h11: A pure-Python HTTP/1.1 protocol library

h11 is an HTTP/1.1 protocol library written in Python, heavily inspired by hyper-h2.

h11’s goal is to be a simple, robust, complete, and non-hacky implementation of the first “chapter” of the HTTP/1.1 spec: RFC 7230: HTTP/1.1 Message Syntax and Routing. That is, it mostly focuses on implementing HTTP at the level of taking bytes on and off the wire, and the headers related to that, and tries to be picky about spec conformance when possible. It doesn’t know about higher-level concerns like URL routing, conditional GETs, cross-origin cookie policies, or content negotiation. But it does know how to take care of framing, cross-version differences in keep-alive handling, and the “obsolete line folding” rule, and to use bounded time and space to process even pathological / malicious input, so that you can focus your energies on the hard / interesting parts for your application. And it tries to support the full specification in the sense that any useful HTTP/1.1 conformant application should be able to use h11.

This is a “bring-your-own-I/O” protocol library; like h2, it contains no I/O code whatsoever. This means you can hook h11 up to your favorite network API, and that could be anything you want: synchronous, threaded, asynchronous, or your own implementation of RFC 6214 – h11 won’t judge you. This is h11’s main feature compared to the current state of the art, where every HTTP library is tightly bound to a particular network framework, and every time a new network API comes along then someone has to start over reimplementing the entire HTTP stack from scratch. We highly recommend Cory Benfield’s excellent blog post about the advantages of this approach.

This also means that h11 is not immediately useful out of the box: it’s a toolkit for building programs that speak HTTP, not something that could directly replace requests or twisted.web or whatever. But h11 makes it much easier to implement something like requests or twisted.web.

Vital statistics

Contents

Getting started: Writing your own HTTP/1.1 client

h11 can be used to implement both HTTP/1.1 clients and servers. To give a flavor for how the API works, we’ll demonstrate a small client.

HTTP basics

An HTTP interaction always starts with a client sending a request, optionally some data (e.g., a POST body); and then the server responds with a response and optionally some data (e.g. the requested document). Requests and responses have some data associated with them: for requests, this is a method (e.g. GET), a target (e.g. /index.html), and a collection of headers (e.g. User-agent: demo-clent). For responses, it’s a status code (e.g. 404 Not Found) and a collection of headers.

Of course, as far as the network is concerned, there’s no such thing as “requests” and “responses” – there’s just bytes being sent from one computer to another. Let’s see what this looks like, by fetching https://httpbin.org/xml:

In [1]: import ssl, socket

In [2]: ctx = ssl.create_default_context()

In [3]: sock = ctx.wrap_socket(socket.create_connection(("httpbin.org", 443)),
   ...:                        server_hostname="httpbin.org")
   ...: 

# Send request
In [4]: sock.sendall(b"GET /xml HTTP/1.1\r\nhost: httpbin.org\r\n\r\n")

# Read response
In [5]: response_data = sock.recv(1024)

# Let's see what we got!
In [6]: print(response_data)
b'HTTP/1.1 200 OK\r\nDate: Sun, 25 Sep 2022 15:37:59 GMT\r\nContent-Type: application/xml\r\nContent-Length: 522\r\nConnection: keep-alive\r\nServer: gunicorn/19.9.0\r\nAccess-Control-Allow-Origin: *\r\nAccess-Control-Allow-Credentials: true\r\n\r\n<?xml version=\'1.0\' encoding=\'us-ascii\'?>\n\n<!--  A SAMPLE set of slides  -->\n\n<slideshow \n    title="Sample Slide Show"\n    date="Date of publication"\n    author="Yours Truly"\n    >\n\n    <!-- TITLE SLIDE -->\n    <slide type="all">\n      <title>Wake up to WonderWidgets!</title>\n    </slide>\n\n    <!-- OVERVIEW -->\n    <slide type="all">\n        <title>Overview</title>\n        <item>Why <em>WonderWidgets</em> are great</item>\n        <item/>\n        <item>Who <em>buys</em> WonderWidgets</item>\n    </slide>\n\n</slideshow>'

Warning

If you try to reproduce these examples interactively, then you’ll have the most luck if you paste them in all at once. Remember we’re talking to a remote server here – if you type them in one at a time, and you’re too slow, then the server might give up on waiting for you and close the connection. One way to recognize that this has happened is if response_data comes back as an empty string, or later on when we’re working with h11 this might cause errors that mention ConnectionClosed.

So that’s, uh, very convenient and readable. It’s a little more understandable if we print the bytes as text:

In [7]: print(response_data.decode("ascii"))
HTTP/1.1 200 OK
Date: Sun, 25 Sep 2022 15:37:59 GMT
Content-Type: application/xml
Content-Length: 522
Connection: keep-alive
Server: gunicorn/19.9.0
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true

<?xml version='1.0' encoding='us-ascii'?>

<!--  A SAMPLE set of slides  -->

<slideshow 
    title="Sample Slide Show"
    date="Date of publication"
    author="Yours Truly"
    >

    <!-- TITLE SLIDE -->
    <slide type="all">
      <title>Wake up to WonderWidgets!</title>
    </slide>

    <!-- OVERVIEW -->
    <slide type="all">
        <title>Overview</title>
        <item>Why <em>WonderWidgets</em> are great</item>
        <item/>
        <item>Who <em>buys</em> WonderWidgets</item>
    </slide>

</slideshow>

Here we can see the status code at the top (200, which is the code for “OK”), followed by the headers, followed by the data (a silly little XML document). But we can already see that working with bytes by hand like this is really cumbersome. What we need to do is to move up to a higher level of abstraction.

This is what h11 does. Instead of talking in bytes, it lets you talk in high-level HTTP “events”. To see what this means, let’s repeat the above exercise, but using h11. We start by making a TLS connection like before, but now we’ll also import h11, and create a h11.Connection object:

In [8]: import ssl, socket

In [9]: import h11

In [10]: ctx = ssl.create_default_context()

In [11]: sock = ctx.wrap_socket(socket.create_connection(("httpbin.org", 443)),
   ....:                        server_hostname="httpbin.org")
   ....: 

In [12]: conn = h11.Connection(our_role=h11.CLIENT)

Next, to send an event to the server, there are three steps we have to take. First, we create an object representing the event we want to send – in this case, a h11.Request:

In [13]: request = h11.Request(method="GET",
   ....:                       target="/xml",
   ....:                       headers=[("Host", "httpbin.org")])
   ....:

Next, we pass this to our connection’s send() method, which gives us back the bytes corresponding to this message:

In [14]: bytes_to_send = conn.send(request)

And then we send these bytes across the network:

In [15]: sock.sendall(bytes_to_send)

There’s nothing magical here – these are the same bytes that we sent up above:

In [16]: bytes_to_send
Out[16]: b'GET /xml HTTP/1.1\r\nHost: httpbin.org\r\n\r\n'

Why doesn’t h11 go ahead and send the bytes for you? Because it’s designed to be usable no matter what socket API you’re using – doesn’t matter if it’s synchronous like this, asynchronous, callback-based, whatever; if you can read and write bytes from the network, then you can use h11.

In this case, we’re not quite done yet – we have to send another event to tell the other side that we’re finished, which we do by sending an EndOfMessage event:

In [17]: end_of_message_bytes_to_send = conn.send(h11.EndOfMessage())

In [18]: sock.sendall(end_of_message_bytes_to_send)

Of course, it turns out that in this case, the HTTP/1.1 specification tells us that any request that doesn’t contain either a Content-Length or Transfer-Encoding header automatically has a 0 length body, and h11 knows that, and h11 knows that the server knows that, so it actually encoded the EndOfMessage event as the empty string:

In [19]: end_of_message_bytes_to_send
Out[19]: b''

But there are other cases where it might not, depending on what headers are set, what message is being responded to, the HTTP version of the remote peer, etc. etc. So for consistency, h11 requires that you always finish your messages by sending an explicit EndOfMessage event; then it keeps track of the details of what that actually means in any given situation, so that you don’t have to.

Finally, we have to read the server’s reply. By now you can probably guess how this is done, at least in the general outline: we read some bytes from the network, then we hand them to the connection (using Connection.receive_data()) and it converts them into events (using Connection.next_event()).

In [20]: bytes_received = sock.recv(1024)

In [21]: conn.receive_data(bytes_received)

In [22]: conn.next_event()
Out[22]: Response(headers=<Headers([(b'date', b'Sun, 25 Sep 2022 15:37:59 GMT'), (b'content-type', b'application/xml'), (b'content-length', b'522'), (b'connection', b'keep-alive'), (b'server', b'gunicorn/19.9.0'), (b'access-control-allow-origin', b'*'), (b'access-control-allow-credentials', b'true')])>, http_version=b'1.1', reason=b'OK', status_code=200)

In [23]: conn.next_event()
Out[23]: Data(data=bytearray(b'<?xml version=\'1.0\' encoding=\'us-ascii\'?>\n\n<!--  A SAMPLE set of slides  -->\n\n<slideshow \n    title="Sample Slide Show"\n    date="Date of publication"\n    author="Yours Truly"\n    >\n\n    <!-- TITLE SLIDE -->\n    <slide type="all">\n      <title>Wake up to WonderWidgets!</title>\n    </slide>\n\n    <!-- OVERVIEW -->\n    <slide type="all">\n        <title>Overview</title>\n        <item>Why <em>WonderWidgets</em> are great</item>\n        <item/>\n        <item>Who <em>buys</em> WonderWidgets</item>\n    </slide>\n\n</slideshow>'), chunk_start=False, chunk_end=False)

In [24]: conn.next_event()
Out[24]: EndOfMessage(headers=<Headers([])>)

(Remember, if you’re following along and get an error here mentioning ConnectionClosed, then try again, but going through the steps faster!)

Here the server sent us three events: a Response object, which is similar to the Request object that we created earlier and has the response’s status code (200 OK) and headers; a Data object containing the response data; and another EndOfMessage object. This similarity between what we send and what we receive isn’t accidental: if we were using h11 to write an HTTP server, then these are the objects we would have created and passed to send() – h11 in client and server mode has an API that’s almost exactly symmetric.

One thing we have to deal with, though, is that an entire response doesn’t always arrive in a single call to socket.recv() – sometimes the network will decide to trickle it in at its own pace, in multiple pieces. Let’s try that again:

In [25]: import ssl, socket

In [26]: import h11

In [27]: ctx = ssl.create_default_context()

In [28]: sock = ctx.wrap_socket(socket.create_connection(("httpbin.org", 443)),
   ....:                        server_hostname="httpbin.org")
   ....: 

In [29]: conn = h11.Connection(our_role=h11.CLIENT)

In [30]: request = h11.Request(method="GET",
   ....:                       target="/xml",
   ....:                       headers=[("Host", "httpbin.org")])
   ....: 

In [31]: sock.sendall(conn.send(request))

and this time, we’ll read in chunks of 200 bytes, to see how h11 handles it:

In [32]: bytes_received = sock.recv(200)

In [33]: conn.receive_data(bytes_received)

In [34]: conn.next_event()
Out[34]: NEED_DATA

NEED_DATA is a special value that indicates that we, well, need more data. h11 has buffered the first chunk of data; let’s read some more:

In [35]: bytes_received = sock.recv(200)

In [36]: conn.receive_data(bytes_received)

In [37]: conn.next_event()
Out[37]: Response(headers=<Headers([(b'date', b'Sun, 25 Sep 2022 15:37:59 GMT'), (b'content-type', b'application/xml'), (b'content-length', b'522'), (b'connection', b'keep-alive'), (b'server', b'gunicorn/19.9.0'), (b'access-control-allow-origin', b'*'), (b'access-control-allow-credentials', b'true')])>, http_version=b'1.1', reason=b'OK', status_code=200)

Now it’s managed to read a complete Request.

A basic client object

Now let’s use what we’ve learned to wrap up our socket and Connection into a single object with some convenience methods:

import socket, ssl
import h11

class MyHttpClient:
    def __init__(self, host, port):
        self.sock = socket.create_connection((host, port))
        if port == 443:
            ctx = ssl.create_default_context()
            self.sock = ctx.wrap_socket(self.sock, server_hostname=host)
        self.conn = h11.Connection(our_role=h11.CLIENT)

    def send(self, *events):
        for event in events:
            data = self.conn.send(event)
            if data is None:
                # event was a ConnectionClosed(), meaning that we won't be
                # sending any more data:
                self.sock.shutdown(socket.SHUT_WR)
            else:
                self.sock.sendall(data)

    # max_bytes_per_recv intentionally set low for pedagogical purposes
    def next_event(self, max_bytes_per_recv=200):
        while True:
            # If we already have a complete event buffered internally, just
            # return that. Otherwise, read some data, add it to the internal
            # buffer, and then try again.
            event = self.conn.next_event()
            if event is h11.NEED_DATA:
                self.conn.receive_data(self.sock.recv(max_bytes_per_recv))
                continue
            return event

And then we can send requests:

In [38]: client = MyHttpClient("httpbin.org", 443)

In [39]: client.send(h11.Request(method="GET", target="/xml",
   ....:                         headers=[("Host", "httpbin.org")]))
   ....: 

In [40]: client.send(h11.EndOfMessage())

And read back the events:

In [41]: client.next_event()
Out[41]: Response(headers=<Headers([(b'date', b'Sun, 25 Sep 2022 15:37:59 GMT'), (b'content-type', b'application/xml'), (b'content-length', b'522'), (b'connection', b'keep-alive'), (b'server', b'gunicorn/19.9.0'), (b'access-control-allow-origin', b'*'), (b'access-control-allow-credentials', b'true')])>, http_version=b'1.1', reason=b'OK', status_code=200)

In [42]: client.next_event()
Out[42]: Data(data=bytearray(b'<?xml version=\'1.0\' encoding=\'us-ascii\'?>\n\n<!--  A SAMPLE set of slides  -->\n\n<slideshow \n    title="Sample Slide Show"\n    date="Date of publication"\n    author="Yours Tr'), chunk_start=False, chunk_end=False)

Note here that we received a Data event that only has part of the response body – this is another consequence of our reading in small chunks. h11 tries to buffer as little as it can, so it streams out data as it arrives, which might mean that a message body might be split up into multiple Data events. (Of course, if you’re the one sending data, you can do the same thing: instead of buffering all your data in one giant Data event, you can send multiple Data events yourself to stream the data out incrementally; just make sure that you set the appropriate Content-Length / Transfer-Encoding headers.) If we keep reading, we’ll see more Data events, and then eventually the EndOfMessage:

In [43]: client.next_event()
Out[43]: Data(data=bytearray(b'uly"\n    >\n\n    <!-- TITLE SLIDE -->\n    <slide type="all">\n      <title>Wake up to WonderWidgets!</title>\n    </slide>\n\n    <!-- OVERVIEW -->\n    <slide type="all">\n        <title>Overview</title>\n  '), chunk_start=False, chunk_end=False)

In [44]: client.next_event()
Out[44]: Data(data=bytearray(b'      <item>Why <em>WonderWidgets</em> are great</item>\n        <item/>\n        <item>Who <em>buys</em> WonderWidgets</item>\n    </slide>\n\n</slideshow>'), chunk_start=False, chunk_end=False)

In [45]: client.next_event()
Out[45]: EndOfMessage(headers=<Headers([])>)

Now we can see why EndOfMessage is so important – otherwise, we can’t tell when we’ve received the end of the data. And since that’s the end of this response, the server won’t send us anything more until we make another request – if we try, then the socket read will just hang forever, unless we set a timeout or interrupt it:

In [46]: client.sock.settimeout(2)

In [47]: client.next_event()
---------------------------------------------------------------------------
timeout                                   Traceback (most recent call last)
<ipython-input-47-c552b3004411> in <module>
----> 1 client.next_event()

<string> in next_event(self, max_bytes_per_recv)

~/.pyenv/versions/3.7.9/lib/python3.7/ssl.py in recv(self, buflen, flags)
   1054                     "non-zero flags not allowed in calls to recv() on %s" %
   1055                     self.__class__)
-> 1056             return self.read(buflen)
   1057         else:
   1058             return super().recv(buflen, flags)

~/.pyenv/versions/3.7.9/lib/python3.7/ssl.py in read(self, len, buffer)
    929                 return self._sslobj.read(len, buffer)
    930             else:
--> 931                 return self._sslobj.read(len)
    932         except SSLError as x:
    933             if x.args[0] == SSL_ERROR_EOF and self.suppress_ragged_eofs:

timeout: The read operation timed out

Keep-alive

For some servers, we’d have to stop here, because they require a new connection for every request/response. But, this server is smarter than that – it supports keep-alive, so we can re-use this connection to send another request. There’s a few ways we can tell. First, if it didn’t, then it would have closed the connection already, and we would have gotten a ConnectionClosed event on our last call to next_event(). We can also tell by checking h11’s internal idea of what state the two sides of the conversation are in:

In [48]: client.conn.our_state, client.conn.their_state
Out[48]: (DONE, DONE)

If the server didn’t support keep-alive, then these would be MUST_CLOSE and either MUST_CLOSE or CLOSED, respectively (depending on whether we’d seen the socket actually close yet). DONE / DONE, on the other hand, means that this request/response cycle has totally finished, but the connection itself is still viable, and we can start over and send a new request on this same connection.

To do this, we tell h11 to get ready (this is needed as a safety measure to make sure different requests/responses on the same connection don’t get accidentally mixed up):

In [49]: client.conn.start_next_cycle()

This resets both sides back to their initial IDLE state, allowing us to send another Request:

In [50]: client.conn.our_state, client.conn.their_state
Out[50]: (IDLE, IDLE)

In [51]: client.send(h11.Request(method="GET", target="/get",
   ....:                         headers=[("Host", "httpbin.org")]))
   ....: 

In [52]: client.send(h11.EndOfMessage())

In [53]: client.next_event()
Out[53]: Response(headers=<Headers([(b'date', b'Sun, 25 Sep 2022 15:38:01 GMT'), (b'content-type', b'application/json'), (b'content-length', b'199'), (b'connection', b'keep-alive'), (b'server', b'gunicorn/19.9.0'), (b'access-control-allow-origin', b'*'), (b'access-control-allow-credentials', b'true')])>, http_version=b'1.1', reason=b'OK', status_code=200)

What’s next?

Here’s some ideas of things you might try:

  • Adapt the above examples to make a POST request. (Don’t forget to set the Content-Length header – but don’t worry, if you do forget, then h11 will give you an error when you try to send data):

    client.send(h11.Request(method="POST", target="/post",
                        headers=[("Host", "httpbin.org"),
                                 ("Content-Length", "10")]))
client.send(h11.Data(data=b"1234567890"))
client.send(h11.EndOfMessage())

  • Experiment with what happens if you try to violate the HTTP protocol by sending a Response as a client, or sending two Requests in a row.

  • Write your own basic http_get function that takes a URL, parses out the host/port/path, then connects to the server, does a GET request, and then collects up all the resulting Data objects, concatenates their payloads, and returns it.

  • Adapt the above code to use your favorite non-blocking API

  • Use h11 to write a simple HTTP server. (If you get stuck, here’s an example.)

And of course, you’ll want to read the API documentation for all the details.

API documentation

h11 has a fairly small public API, with all public symbols available directly at the top level:

In [1]: import h11

In [2]: h11.<TAB>
h11.CLIENT                 h11.MUST_CLOSE
h11.CLOSED                 h11.NEED_DATA
h11.Connection             h11.PAUSED
h11.ConnectionClosed       h11.PRODUCT_ID
h11.Data                   h11.ProtocolError
h11.DONE                   h11.RemoteProtocolError
h11.EndOfMessage           h11.Request
h11.ERROR                  h11.Response
h11.IDLE                   h11.SEND_BODY
h11.InformationalResponse  h11.SEND_RESPONSE
h11.LocalProtocolError     h11.SERVER
h11.MIGHT_SWITCH_PROTOCOL  h11.SWITCHED_PROTOCOL

These symbols fall into three main categories: event classes, special constants used to track different connection states, and the Connection class itself. We’ll describe them in that order.

Events

Events are the core of h11: the whole point of h11 is to let you think about HTTP transactions as being a series of events sent back and forth between a client and a server, instead of thinking in terms of bytes.

All events behave in essentially similar ways. Let’s take Request as an example. Like all events, this is a “final” class – you cannot subclass it. And like all events, it has several fields. For Request, there are four of them: method, target, headers, and http_version. http_version defaults to b"1.1"; the rest have no default, so to create a Request you have to specify their values:

In [3]: req = h11.Request(method="GET",
   ...:                   target="/",
   ...:                   headers=[("Host", "example.com")])
   ...:

Event constructors accept only keyword arguments, not positional arguments.

Events have a useful repr:

In [4]: req
Out[4]: Request(method=b'GET', headers=<Headers([(b'host', b'example.com')])>, target=b'/', http_version=b'1.1')

And their fields are available as regular attributes:

In [5]: req.method
Out[5]: b'GET'

In [6]: req.target
Out[6]: b'/'

In [7]: req.headers
Out[7]: <Headers([(b'host', b'example.com')])>

In [8]: req.http_version
Out[8]: b'1.1'

Notice that these attributes have been normalized to byte-strings. In general, events normalize and validate their fields when they’re constructed. Some of these normalizations and checks are specific to a particular event – for example, Request enforces RFC 7230’s requirement that HTTP/1.1 requests must always contain a "Host" header:

# HTTP/1.0 requests don't require a Host: header
In [9]: h11.Request(method="GET", target="/", headers=[], http_version="1.0")
Out[9]: Request(method=b'GET', headers=<Headers([])>, target=b'/', http_version=b'1.0')

# But HTTP/1.1 requests do
In [10]: h11.Request(method="GET", target="/", headers=[])
---------------------------------------------------------------------------
LocalProtocolError                        Traceback (most recent call last)
<ipython-input-10-bd1fad8653ff> in <module>
----> 1 h11.Request(method="GET", target="/", headers=[])

~/checkouts/readthedocs.org/user_builds/h11/envs/stable/lib/python3.7/site-packages/h11-0.14.0-py3.7.egg/h11/_events.py in __init__(self, method, headers, target, http_version, _parsed)
    115                 host_count += 1
    116         if self.http_version == b"1.1" and host_count == 0:
--> 117             raise LocalProtocolError("Missing mandatory Host: header")
    118         if host_count > 1:
    119             raise LocalProtocolError("Found multiple Host: headers")

LocalProtocolError: Missing mandatory Host: header

This helps protect you from accidentally violating the protocol, and also helps protect you from remote peers who attempt to violate the protocol.

A few of these normalization rules are standard across multiple events, so we document them here:

headers: In h11, headers are represented internally as a list of (name, value) pairs, where name and value are both byte-strings, name is always lowercase, and name and value are both guaranteed not to have any leading or trailing whitespace. When constructing an event, we accept any iterable of pairs like this, and will automatically convert native strings containing ascii or bytes-like objects to byte-strings and convert names to lowercase:

In [11]: original_headers = [("HOST", bytearray(b"Example.Com"))]

In [12]: req = h11.Request(method="GET", target="/", headers=original_headers)

In [13]: original_headers
Out[13]: [('HOST', bytearray(b'Example.Com'))]

In [14]: req.headers
Out[14]: <Headers([(b'host', b'Example.Com')])>

If any names are detected with leading or trailing whitespace, then this is an error (“in the past, differences in the handling of such whitespace have led to security vulnerabilities” – RFC 7230). We also check for certain other protocol violations, e.g. it’s always illegal to have a newline inside a header value, and Content-Length: hello is an error because Content-Length should always be an integer. We may add additional checks in the future.

While we make sure to expose header names as lowercased bytes, we also preserve the original header casing that is used. Compliant HTTP agents should always treat headers in a case insensitive manner, but this may not always be the case. When sending bytes over the wire we send headers preserving whatever original header casing was used.

It is possible to access the headers in their raw original casing, which may be useful for some user output or debugging purposes.

In [15]: original_headers = [("Host", "example.com")]

In [16]: req = h11.Request(method="GET", target="/", headers=original_headers)

In [17]: req.headers.raw_items()
Out[17]: [(b'Host', b'example.com')]

It’s not just headers we normalize to being byte-strings: the same type-conversion logic is also applied to the Request.method and Request.target field, and – for consistency – all http_version fields. In particular, we always represent HTTP version numbers as byte-strings like b"1.1". Bytes-like objects and native strings will be automatically converted to byte strings. Note that the HTTP standard specifically guarantees that all HTTP version numbers will consist of exactly two digits separated by a dot, so comparisons like req.http_version < b"1.1" are safe and valid.

When manually constructing an event, you generally shouldn’t specify http_version, because it defaults to b"1.1", and if you attempt to override this to some other value then Connection.send() will reject your event – h11 only speaks HTTP/1.1. But it does understand other versions of HTTP, so you might receive events with other http_version values from remote peers.

Here’s the complete set of events supported by h11:

class h11.Request(*, method: Union[bytes, str], headers: Union[h11._headers.Headers, List[Tuple[bytes, bytes]], List[Tuple[str, str]]], target: Union[bytes, str], http_version: Union[bytes, str] = b'1.1', _parsed: bool = False)[source]

The beginning of an HTTP request.

Fields:

method

An HTTP method, e.g. b"GET" or b"POST". Always a byte string. Bytes-like objects and native strings containing only ascii characters will be automatically converted to byte strings.

target

The target of an HTTP request, e.g. b"/index.html", or one of the more exotic formats described in RFC 7320, section 5.3. Always a byte string. Bytes-like objects and native strings containing only ascii characters will be automatically converted to byte strings.

headers

Request headers, represented as a list of (name, value) pairs. See the header normalization rules for details.

http_version

The HTTP protocol version, represented as a byte string like b"1.1". See the HTTP version normalization rules for details.

class h11.InformationalResponse(*, headers: Union[h11._headers.Headers, List[Tuple[bytes, bytes]], List[Tuple[str, str]]], status_code: int, http_version: Union[bytes, str] = b'1.1', reason: Union[bytes, str] = b'', _parsed: bool = False)[source]

An HTTP informational response.

Fields:

status_code

The status code of this response, as an integer. For an InformationalResponse, this is always in the range [100, 200).

headers

Request headers, represented as a list of (name, value) pairs. See the header normalization rules for details.

http_version

The HTTP protocol version, represented as a byte string like b"1.1". See the HTTP version normalization rules for details.

reason

The reason phrase of this response, as a byte string. For example: b"OK", or b"Not Found".

class h11.Response(*, headers: Union[h11._headers.Headers, List[Tuple[bytes, bytes]], List[Tuple[str, str]]], status_code: int, http_version: Union[bytes, str] = b'1.1', reason: Union[bytes, str] = b'', _parsed: bool = False)[source]

The beginning of an HTTP response.

Fields:

status_code

The status code of this response, as an integer. For an Response, this is always in the range [200, 1000).

headers

Request headers, represented as a list of (name, value) pairs. See the header normalization rules for details.

http_version

The HTTP protocol version, represented as a byte string like b"1.1". See the HTTP version normalization rules for details.

reason

The reason phrase of this response, as a byte string. For example: b"OK", or b"Not Found".

class h11.Data(data: bytes, chunk_start: bool = False, chunk_end: bool = False)[source]

Part of an HTTP message body.

Fields:

data

A bytes-like object containing part of a message body. Or, if using the combine=False argument to Connection.send(), then any object that your socket writing code knows what to do with, and for which calling len() returns the number of bytes that will be written – see Support for sendfile() for details.

chunk_start

A marker that indicates whether this data object is from the start of a chunked transfer encoding chunk. This field is ignored when when a Data event is provided to Connection.send(): it is only valid on events emitted from Connection.next_event(). You probably shouldn’t use this attribute at all; see Chunked Transfer Encoding Delimiters for details.

chunk_end

A marker that indicates whether this data object is the last for a given chunked transfer encoding chunk. This field is ignored when when a Data event is provided to Connection.send(): it is only valid on events emitted from Connection.next_event(). You probably shouldn’t use this attribute at all; see Chunked Transfer Encoding Delimiters for details.

class h11.EndOfMessage(*, headers: Union[h11._headers.Headers, List[Tuple[bytes, bytes]], List[Tuple[str, str]], None] = None, _parsed: bool = False)[source]

The end of an HTTP message.

Fields:

headers

Default value: []

Any trailing headers attached to this message, represented as a list of (name, value) pairs. See the header normalization rules for details.

Must be empty unless Transfer-Encoding: chunked is in use.

class h11.ConnectionClosed[source]

This event indicates that the sender has closed their outgoing connection.

Note that this does not necessarily mean that they can’t receive further data, because TCP connections are composed to two one-way channels which can be closed independently. See Closing connections for details.

No fields.

The state machine

Now that you know what the different events are, the next question is: what can you do with them?

A basic HTTP request/response cycle looks like this:

And once that’s finished, both sides either close the connection, or they go back to the top and re-use it for another request/response cycle.

To coordinate this interaction, the h11 Connection object maintains several state machines: one that tracks what the client is doing, one that tracks what the server is doing, and a few more tiny ones to track whether keep-alive is enabled and whether the client has proposed to switch protocols. h11 always keeps track of all of these state machines, regardless of whether it’s currently playing the client or server role.

The state machines look like this (click on each to expand):

client-image
server-image
special-image

If you squint at the first two diagrams, you can see the client’s IDLE -> SEND_BODY -> DONE path and the server’s IDLE -> SEND_RESPONSE -> SEND_BODY -> DONE path, which encode the basic sequence of events we described above. But there’s a fair amount of other stuff going on here as well.

The first thing you should notice is the different colors. These correspond to the different ways that our state machines can change state.

  • Dark blue arcs are event-triggered transitions: if we’re in state A, and this event happens, when we switch to state B. For the client machine, these transitions always happen when the client sends an event. For the server machine, most of them involve the server sending an event, except that the server also goes from IDLE -> SEND_RESPONSE when the client sends a Request.
  • Green arcs are state-triggered transitions: these are somewhat unusual, and are used to couple together the different state machines – if, at any moment, one machine is in state A and another machine is in state B, then the first machine immediately transitions to state C. For example, if the CLIENT machine is in state DONE, and the SERVER machine is in the CLOSED state, then the CLIENT machine transitions to MUST_CLOSE. And the same thing happens if the CLIENT machine is in the state DONE and the keep-alive machine is in the state disabled.
  • There are also two purple arcs labeled start_next_cycle(): these correspond to an explicit method call documented below.

Here’s why we have all the stuff in those diagrams above, beyond what’s needed to handle the basic request/response cycle:

  • Server sending a Response directly from IDLE: This is used for error responses, when the client’s request never arrived (e.g. 408 Request Timed Out) or was unparseable gibberish (400 Bad Request) and thus didn’t register with our state machine as a real Request.
  • The transitions involving MUST_CLOSE and CLOSE: keep-alive and shutdown handling; see Re-using a connection: keep-alive and pipelining and Closing connections.
  • The transitions involving MIGHT_SWITCH_PROTOCOL and SWITCHED_PROTOCOL: See Switching protocols.
  • That weird ERROR state hanging out all lonely on the bottom: to avoid cluttering the diagram, we don’t draw any arcs coming into this node, but that doesn’t mean it can’t be entered. In fact, it can be entered from any state: if any exception occurs while trying to send/receive data, then the corresponding machine will transition directly to this state. Once there, though, it can never leave – that part of the diagram is accurate. See Error handling.

And finally, note that in these diagrams, all the labels that are in italics are informal English descriptions of things that happen in the code, while the labels in upright text correspond to actual objects in the public API. You’ve already seen the event objects like Request and Response; there are also a set of opaque sentinel values that you can use to track and query the client and server’s states.

Special constants

h11 exposes some special constants corresponding to the different states in the client and server state machines described above. The complete list is:

h11.IDLE[source]
h11.SEND_RESPONSE[source]
h11.SEND_BODY[source]
h11.DONE[source]
h11.MUST_CLOSE[source]
h11.CLOSED[source]
h11.MIGHT_SWITCH_PROTOCOL[source]
h11.SWITCHED_PROTOCOL[source]
h11.ERROR[source]

For example, we can see that initially the client and server start in state IDLE / IDLE:

In [18]: conn = h11.Connection(our_role=h11.CLIENT)

In [19]: conn.states
Out[19]: {CLIENT: IDLE, SERVER: IDLE}

And then if the client sends a Request, then the client switches to state SEND_BODY, while the server switches to state SEND_RESPONSE:

In [20]: conn.send(h11.Request(method="GET", target="/", headers=[("Host", "example.com")]));

In [21]: conn.states
Out[21]: {CLIENT: SEND_BODY, SERVER: SEND_RESPONSE}

And we can test these values directly using constants like SEND_BODY:

In [22]: conn.states[h11.CLIENT] is h11.SEND_BODY
Out[22]: True

This shows how the Connection type tracks these state machines and lets you query their current state.

The above also showed the special constants that can be used to indicate the two different roles that a peer can play in an HTTP connection:

h11.CLIENT[source]
h11.SERVER[source]

And finally, there are also two special constants that can be returned from Connection.next_event():

h11.NEED_DATA[source]
h11.PAUSED[source]

All of these behave the same, and their behavior is modeled after None: they’re opaque singletons, their __repr__() is their name, and you compare them with is.

Finally, h11’s constants have a quirky feature that can sometimes be useful: they are instances of themselves.

In [23]: type(h11.NEED_DATA) is h11.NEED_DATA
Out[23]: True

In [24]: type(h11.PAUSED) is h11.PAUSED
Out[24]: True

The main application of this is that when handling the return value from Connection.next_event(), which is sometimes an instance of an event class and sometimes NEED_DATA or PAUSED, you can always call type(event) to get something useful to dispatch one, using e.g. a handler table, functools.singledispatch(), or calling getattr(some_object, "handle_" + type(event).__name__). Not that this kind of dispatch-based strategy is always the best approach – but the option is there if you want it.

The Connection object

class h11.Connection(our_role: Type[h11._util.Sentinel], max_incomplete_event_size: int = 16384)[source]

An object encapsulating the state of an HTTP connection.

Parameters:
  • our_role – If you’re implementing a client, pass h11.CLIENT. If you’re implementing a server, pass h11.SERVER.
  • max_incomplete_event_size (int) – The maximum number of bytes we’re willing to buffer of an incomplete event. In practice this mostly sets a limit on the maximum size of the request/response line + headers. If this is exceeded, then next_event() will raise RemoteProtocolError.
receive_data(data: bytes) → None[source]

Add data to our internal receive buffer.

This does not actually do any processing on the data, just stores it. To trigger processing, you have to call next_event().

Parameters:

data (bytes-like object) –

The new data that was just received.

Special case: If data is an empty byte-string like b"", then this indicates that the remote side has closed the connection (end of file). Normally this is convenient, because standard Python APIs like file.read() or socket.recv() use b"" to indicate end-of-file, while other failures to read are indicated using other mechanisms like raising TimeoutError. When using such an API you can just blindly pass through whatever you get from read to receive_data(), and everything will work.

But, if you have an API where reading an empty string is a valid non-EOF condition, then you need to be aware of this and make sure to check for such strings and avoid passing them to receive_data().
Returns:

Nothing, but after calling this you should call next_event() to parse the newly received data.
Raises:

RuntimeError – Raised if you pass an empty data, indicating EOF, and then pass a non-empty data, indicating more data that somehow arrived after the EOF.

(Calling receive_data(b"") multiple times is fine, and equivalent to calling it once.)
next_event() → Union[h11._events.Event, Type[h11._connection.NEED_DATA], Type[h11._connection.PAUSED]][source]

Parse the next event out of our receive buffer, update our internal state, and return it.

This is a mutating operation – think of it like calling next() on an iterator.

Returns:One of three things:
  1. An event object – see Events.
  2. The special constant NEED_DATA, which indicates that you need to read more data from your socket and pass it to receive_data() before this method will be able to return any more events.
  3. The special constant PAUSED, which indicates that we are not in a state where we can process incoming data (usually because the peer has finished their part of the current request/response cycle, and you have not yet called start_next_cycle()). See Flow control for details.
Raises:RemoteProtocolError – The peer has misbehaved. You should close the connection (possibly after sending some kind of 4xx response).

Once this method returns ConnectionClosed once, then all subsequent calls will also return ConnectionClosed.

If this method raises any exception besides RemoteProtocolError then that’s a bug – if it happens please file a bug report!

If this method raises any exception then it also sets Connection.their_state to ERROR – see Error handling for discussion.

send(event: h11._events.Event) → Optional[bytes][source]

Convert a high-level event into bytes that can be sent to the peer, while updating our internal state machine.

Parameters:event – The event to send.
Returns:If type(event) is ConnectionClosed, then returns None. Otherwise, returns a bytes-like object.
Raises:LocalProtocolError – Sending this event at this time would violate our understanding of the HTTP/1.1 protocol.

If this method raises any exception then it also sets Connection.our_state to ERROR – see Error handling for discussion.

send_with_data_passthrough(event: h11._events.Event) → Optional[List[bytes]][source]

Identical to send(), except that in situations where send() returns a single bytes-like object, this instead returns a list of them – and when sending a Data event, this list is guaranteed to contain the exact object you passed in as Data.data. See Support for sendfile() for discussion.

send_failed() → None[source]

Notify the state machine that we failed to send the data it gave us.

This causes Connection.our_state to immediately become ERROR – see Error handling for discussion.

start_next_cycle() → None[source]

Attempt to reset our connection state for a new request/response cycle.

If both client and server are in DONE state, then resets them both to IDLE state in preparation for a new request/response cycle on this same connection. Otherwise, raises a LocalProtocolError.

See Re-using a connection: keep-alive and pipelining.

our_role

CLIENT if this is a client; SERVER if this is a server.

their_role

SERVER if this is a client; CLIENT if this is a server.

states

A dictionary like:

{CLIENT: <client state>, SERVER: <server state>}

See The state machine for details.

our_state

The current state of whichever role we are playing. See The state machine for details.

their_state

The current state of whichever role we are NOT playing. See The state machine for details.

their_http_version

The version of HTTP that our peer claims to support. None if we haven’t yet received a request/response.

This is preserved by start_next_cycle(), so it can be handy for a client making multiple requests on the same connection: normally you don’t know what version of HTTP the server supports until after you do a request and get a response – so on an initial request you might have to assume the worst. But on later requests on the same connection, the information will be available here.

client_is_waiting_for_100_continue

True if the client sent a request with the Expect: 100-continue header, and is still waiting for a response (i.e., the server has not sent a 100 Continue or any other kind of response, and the client has not gone ahead and started sending the body anyway).

See RFC 7231 section 5.1.1 for details.

they_are_waiting_for_100_continue

True if their_role is CLIENT and client_is_waiting_for_100_continue.

trailing_data

Data that has been received, but not yet processed, represented as a tuple with two elements, where the first is a byte-string containing the unprocessed data itself, and the second is a bool that is True if the receive connection was closed.

See Switching protocols for discussion of why you’d want this.

Error handling

Given the vagaries of networks and the folks on the other side of them, it’s extremely important to be prepared for errors.

Most errors in h11 are signaled by raising one of ProtocolError’s two concrete base classes, LocalProtocolError and RemoteProtocolError:

exception h11.ProtocolError(msg: str, error_status_hint: int = 400)[source]

Exception indicating a violation of the HTTP/1.1 protocol.

This as an abstract base class, with two concrete base classes: LocalProtocolError, which indicates that you tried to do something that HTTP/1.1 says is illegal, and RemoteProtocolError, which indicates that the remote peer tried to do something that HTTP/1.1 says is illegal. See Error handling for details.

In addition to the normal Exception features, it has one attribute:

error_status_hint

This gives a suggestion as to what status code a server might use if this error occurred as part of a request.

For a RemoteProtocolError, this is useful as a suggestion for how you might want to respond to a misbehaving peer, if you’re implementing a server.

For a LocalProtocolError, this can be taken as a suggestion for how your peer might have responded to you if h11 had allowed you to continue.

The default is 400 Bad Request, a generic catch-all for protocol violations.

exception h11.LocalProtocolError(msg: str, error_status_hint: int = 400)[source]
exception h11.RemoteProtocolError(msg: str, error_status_hint: int = 400)[source]

There are four cases where these exceptions might be raised:

  • When trying to instantiate an event object (LocalProtocolError): This indicates that something about your event is invalid. Your event wasn’t constructed, but there are no other consequences – feel free to try again.
  • When calling Connection.start_next_cycle() (LocalProtocolError): This indicates that the connection is not ready to be re-used, because one or both of the peers are not in the DONE state. The Connection object remains usable, and you can try again later.
  • When calling Connection.next_event() (RemoteProtocolError): This indicates that the remote peer has violated our protocol assumptions. This is unrecoverable – we don’t know what they’re doing and we cannot safely proceed. Connection.their_state immediately becomes ERROR, and all further calls to next_event() will also raise RemoteProtocolError. Connection.send() still works as normal, so if you’re implementing a server and this happens then you have an opportunity to send back a 400 Bad Request response. But aside from that, your only real option is to close your socket and make a new connection.
  • When calling Connection.send() or Connection.send_with_data_passthrough() (LocalProtocolError): This indicates that you violated our protocol assumptions. This is also unrecoverable – h11 doesn’t know what you’re doing, its internal state may be inconsistent, and we cannot safely proceed. Connection.our_state immediately becomes ERROR, and all further calls to send() will also raise LocalProtocolError. The only thing you can reasonably due at this point is to close your socket and make a new connection.

So that’s how h11 tells you about errors that it detects. In some cases, it’s also useful to be able to tell h11 about an error that you detected. In particular, the Connection object assumes that after you call Connection.send(), you actually send that data to the remote peer. But sometimes, for one reason or another, this doesn’t actually happen.

Here’s a concrete example. Suppose you’re using h11 to implement an HTTP client that keeps a pool of connections so it can re-use them when possible (see Re-using a connection: keep-alive and pipelining). You take a connection from the pool, and start to do a large upload… but then for some reason this gets cancelled (maybe you have a GUI and a user clicked “cancel”). This can cause h11’s model of this connection to diverge from reality: for example, h11 might think that you successfully sent the full request, because you passed an EndOfMessage object to Connection.send(), but in fact you didn’t, because you never sent the resulting bytes. And then – here’s the really tricky part! – if you’re not careful, you might think that it’s OK to put this connection back into the connection pool and re-use it, because h11 is telling you that a full request/response cycle was completed. But this is wrong; in fact you have to close this connection and open a new one.

The solution is simple: call Connection.send_failed(), and now h11 knows that your send failed. In this case, Connection.our_state immediately becomes ERROR, just like if you had tried to do something that violated the protocol.

Message body framing: Content-Length and all that

There are two different headers that HTTP/1.1 uses to indicate a framing mechanism for request/response bodies: Content-Length and Transfer-Encoding. Our general philosophy is that the way you tell h11 what configuration you want to use is by setting the appropriate headers in your request / response, and then h11 will both pass those headers on to the peer and encode the body appropriately.

Currently, the only supported Transfer-Encoding is chunked.

On requests, this means:

  • No Content-Length or Transfer-Encoding: no body, equivalent to Content-Length: 0.

  • Content-Length: ...: You’re going to send exactly the specified number of bytes. h11 will keep track and signal an error if your EndOfMessage doesn’t happen at the right place.

  • Transfer-Encoding: chunked: You’re going to send a variable / not yet known number of bytes.

    Note 1: only HTTP/1.1 servers are required to support Transfer-Encoding: chunked, and as a client you have to decide whether to send this header before you get to see what protocol version the server is using.

    Note 2: even though HTTP/1.1 servers are required to support Transfer-Encoding: chunked, this doesn’t necessarily mean that they actually do – e.g., applications using Python’s standard WSGI API cannot accept chunked requests.

    Nonetheless, this is the only way to send request where you don’t know the size of the body ahead of time, so if that’s the situation you find yourself in then you might as well try it and hope.

On responses, things are a bit more subtle. There are effectively two cases:

  • Content-Length: ...: You’re going to send exactly the specified number of bytes. h11 will keep track and signal an error if your EndOfMessage doesn’t happen at the right place.

  • Transfer-Encoding: chunked, or, neither framing header is provided: These two cases are handled differently at the wire level, but as far as the application is concerned they provide (almost) exactly the same semantics: in either case, you’ll send a variable / not yet known number of bytes. The difference between them is that Transfer-Encoding: chunked works better (compatible with keep-alive, allows trailing headers, clearly distinguishes between successful completion and network errors), but requires an HTTP/1.1 client; for HTTP/1.0 clients the only option is the no-headers approach where you have to close the socket to indicate completion.

    Since this is (almost) entirely a wire-level-encoding concern, h11 abstracts it: when sending a response you can set either Transfer-Encoding: chunked or leave off both framing headers, and h11 will treat both cases identically: it will automatically pick the best option given the client’s advertised HTTP protocol level.

    You need to watch out for this if you’re using trailing headers (i.e., a non-empty headers attribute on EndOfMessage), since trailing headers are only legal if we actually ended up using Transfer-Encoding: chunked. Trying to send a non-empty set of trailing headers to a HTTP/1.0 client will raise a LocalProtocolError. If this use case is important to you, check Connection.their_http_version to confirm that the client speaks HTTP/1.1 before you attempt to send any trailing headers.

Re-using a connection: keep-alive and pipelining

HTTP/1.1 allows a connection to be re-used for multiple request/response cycles (also known as “keep-alive”). This can make things faster by letting us skip the costly connection setup, but it does create some complexities: we have to keep track of whether a connection is reusable, and when there are multiple requests and responses flowing through the same connection we need to be careful not to get confused about which request goes with which response.

h11 considers a connection to be reusable if, and only if, both sides (a) speak HTTP/1.1 (HTTP/1.0 did have some complex and fragile support for keep-alive bolted on, but h11 currently doesn’t support that – possibly this will be added in the future), and (b) neither side has explicitly disabled keep-alive by sending a Connection: close header.

If you plan to make only a single request or response and then close the connection, you should manually set the Connection: close header in your request/response. h11 will notice and update its state appropriately.

There are also some situations where you are required to send a Connection: close header, e.g. if you are a server talking to a client that doesn’t support keep-alive. You don’t need to worry about these cases – h11 will automatically add this header when necessary. Just worry about setting it when it’s actually something that you’re actively choosing.

If you want to re-use a connection, you have to wait until both the request and the response have been completed, bringing both the client and server to the DONE state. Once this has happened, you can explicitly call Connection.start_next_cycle() to reset both sides back to the IDLE state. This makes sure that the client and server remain synched up.

If keep-alive is disabled for whatever reason – someone set Connection: close, lack of protocol support, one of the sides just unilaterally closed the connection – then the state machines will skip past the DONE state directly to the MUST_CLOSE or CLOSED states. In this case, trying to call start_next_cycle() will raise an error, and the only thing you can legally do is to close this connection and make a new one.

HTTP/1.1 also allows for a more aggressive form of connection re-use, in which a client sends multiple requests in quick succession, and then waits for the responses to stream back in order (“pipelining”). This is generally considered to have been a bad idea, because it makes things like error recovery very complicated.

As a client, h11 does not support pipelining. This is enforced by the structure of the state machine: after sending one Request, you can’t send another until after calling start_next_cycle(), and you can’t call start_next_cycle() until the server has entered the DONE state, which requires reading the server’s full response.

As a server, h11 provides the minimal support for pipelining required to comply with the HTTP/1.1 standard: if the client sends multiple pipelined requests, then we handle the first request until we reach the DONE state, and then next_event() will pause and refuse to parse any more events until the response is completed and start_next_cycle() is called. See the next section for more details.

Flow control

Presumably you know when you want to send things, and the send() interface is very simple: it just immediately returns all the data you need to send for the given event, so you can apply whatever send buffer strategy you want. But reading from the remote peer is a bit trickier: you don’t want to read data from the remote peer if it can’t be processed (i.e., you want to apply backpressure and avoid building arbitrarily large in-memory buffers), and you definitely don’t want to block waiting on data from the remote peer at the same time that it’s blocked waiting for you, because that will cause a deadlock.

One complication here is that if you’re implementing a server, you have to be prepared to handle Requests that have an Expect: 100-continue header. You can read the spec for the full details, but basically what this header means is that after sending the Request, the client plans to pause and wait until they see some response from the server before they send that request’s Data. The server’s response would normally be an InformationalResponse with status 100 Continue, but it could be anything really (e.g. a full Response with a 4xx status code). The crucial thing as a server, though, is that you should never block trying to read a request body if the client is blocked waiting for you to tell them to send the request body.

Fortunately, h11 makes this easy, because it tracks whether the client is in the waiting-for-100-continue state, and exposes this as Connection.they_are_waiting_for_100_continue. So you don’t have to pay attention to the Expect header yourself; you just have to make sure that before you block waiting to read a request body, you execute some code like:

if conn.they_are_waiting_for_100_continue:
    do_send(conn, h11.InformationalResponse(100, headers=[...]))
do_read(...)

In fact, if you’re lazy (and what programmer isn’t?) then you can just do this check before all reads – it’s mandatory before blocking to read a request body, but it’s safe at any time.

And the other thing you want to pay attention to is the special values that next_event() might return: NEED_DATA and PAUSED.

NEED_DATA is what it sounds like: it means that next_event() is guaranteed not to return any more real events until you’ve called receive_data() at least once.

PAUSED is a little more subtle: it means that next_event() is guaranteed not to return any more real events until something else has happened to clear up the paused state. There are three cases where this can happen:

  1. We received a full request/response from the remote peer, and then we received some more data after that. (The main situation where this might happen is a server responding to a pipelining client.) The PAUSED state will go away after you call start_next_cycle().
  2. A successful CONNECT or Upgrade: request has caused the connection to switch to some other protocol (see Switching protocols). This PAUSED state is permanent; you should abandon this Connection and go do whatever it is you’re going to do with your new protocol.
  3. We’re a server, and the client we’re talking to proposed to switch protocols (see Switching protocols), and now is waiting to find out whether their request was successful or not. Once we either accept or deny their request then this will turn into one of the above two states, so you probably don’t need to worry about handling it specially.

Putting all this together –

If your I/O is organized around a “pull” strategy, where your code requests events as its ready to handle them (e.g. classic synchronous code, or asyncio’s await loop.sock_recv(...), or Trio’s streams), then you’ll probably want logic that looks something like:

# Replace do_sendall and do_recv with your I/O code
def get_next_event():
    while True:
        event = conn.next_event()
        if event is h11.NEED_DATA:
            if conn.they_are_waiting_for_100_continue:
                do_sendall(conn, h11.InformationalResponse(100, ...))
            conn.receive_data(do_recv())
            continue
        return event

And then your code that calls this will need to make sure to call it only at appropriate times (e.g., not immediately after receiving EndOfMessage or PAUSED).

If your I/O is organized around a “push” strategy, where the network drives processing (e.g. you’re using Twisted, or implementing an asyncio.Protocol), then you’ll want to internally apply back-pressure whenever you see PAUSED, remove back-pressure when you call start_next_cycle(), and otherwise just deliver events as they arrive. Something like:

class HTTPProtocol(asyncio.Protocol):
    # Save the transport for later -- needed to access the
    # backpressure API.
    def connection_made(self, transport):
        self._transport = transport

    # Internal helper function -- deliver all pending events
    def _deliver_events(self):
        while True:
            event = self.conn.next_event()
            if event is h11.NEED_DATA:
                break
            elif event is h11.PAUSED:
                # Apply back-pressure
                self._transport.pause_reading()
                break
            else:
                self.event_received(event)

    # Called by "someone" whenever new data appears on our socket
    def data_received(self, data):
        self.conn.receive_data(data)
        self._deliver_events()

    # Called by "someone" whenever the peer closes their socket
    def eof_received(self):
        self.conn.receive_data(b"")
        self._deliver_events()
        # asyncio will close our socket unless we return True here.
        return True

    # Called by your code when its ready to start a new
    # request/response cycle
    def start_next_cycle(self):
        self.conn.start_next_cycle()
        # New events might have been buffered internally, and only
        # become deliverable after calling start_next_cycle
        self._deliver_events()
        # Remove back-pressure
        self._transport.resume_reading()

    # Fill in your code here
    def event_received(self, event):
        ...

And your code that uses this will have to remember to check for they_are_waiting_for_100_continue at the appropriate time.

Closing connections

h11 represents a connection shutdown with the special event type ConnectionClosed. You can send this event, in which case send() will simply update the state machine and then return None. You can receive this event, if you call conn.receive_data(b""). (The actual receipt might be delayed if the connection is paused.) It’s safe and legal to call conn.receive_data(b"") multiple times, and once you’ve done this once, then all future calls to receive_data() will also return ConnectionClosed():

In [25]: conn = h11.Connection(our_role=h11.CLIENT)

In [26]: conn.receive_data(b"")

In [27]: conn.receive_data(b"")

In [28]: conn.receive_data(None)

(Or if you try to actually pass new data in after calling conn.receive_data(b""), that will raise an exception.)

h11 is careful about interpreting connection closure in a half-duplex fashion. TCP sockets pretend to be a two-way connection, but really they’re two one-way connections. In particular, it’s possible for one party to shut down their sending connection – which causes the other side to be notified that the connection has closed via the usual socket.recv(...) -> b"" mechanism – while still being able to read from their receiving connection. (On Unix, this is generally accomplished via the shutdown(2) system call.) So, for example, a client could send a request, and then close their socket for writing to indicate that they won’t be sending any more requests, and then read the response. It’s this kind of closure that is indicated by h11’s ConnectionClosed: it means that this party will not be sending any more data – nothing more, nothing less. You can see this reflected in the state machine, in which one party transitioning to CLOSED doesn’t immediately halt the connection, but merely prevents it from continuing for another request/response cycle.

The state machine also indicates that ConnectionClosed events can only happen in certain states. This isn’t true, of course – any party can close their connection at any time, and h11 can’t stop them. But what h11 can do is distinguish between clean and unclean closes. For example, if both sides complete a request/response cycle and then close the connection, that’s a clean closure and everyone will transition to the CLOSED state in an orderly fashion. On the other hand, if one party suddenly closes the connection while they’re in the middle of sending a chunked response body, or when they promised a Content-Length: of 1000 bytes but have only sent 500, then h11 knows that this is a violation of the HTTP protocol, and will raise a ProtocolError. Basically h11 treats an unexpected close the same way it would treat unexpected, uninterpretable data arriving – it lets you know that something has gone wrong.

As a client, the proper way to perform a single request and then close the connection is:

  1. Send a Request with Connection: close
  2. Send the rest of the request body
  3. Read the server’s Response and body
  4. conn.our_state is h11.MUST_CLOSE will now be true. Call conn.send(ConnectionClosed()) and then close the socket. Or really you could just close the socket – the thing calling send will do is raise an error if you’re not in MUST_CLOSE as expected. So it’s between you and your conscience and your code reviewers.

(Technically it would also be legal to shutdown your socket for writing as step 2.5, but this doesn’t serve any purpose and some buggy servers might get annoyed, so it’s not recommended.)

As a server, the proper way to perform a response is:

  1. Send your Response and body
  2. Check if conn.our_state is h11.MUST_CLOSE. This might happen for a variety of reasons; for example, if the response had unknown length and the client speaks only HTTP/1.0, then the client will not consider the connection complete until we issue a close.

You should be particularly careful to take into consideration the following note fromx RFC 7230 section 6.6:

If a server performs an immediate close of a TCP connection, there is a significant risk that the client will not be able to read the last HTTP response. If the server receives additional data from the client on a fully closed connection, such as another request that was sent by the client before receiving the server’s response, the server’s TCP stack will send a reset packet to the client; unfortunately, the reset packet might erase the client’s unacknowledged input buffers before they can be read and interpreted by the client’s HTTP parser.

To avoid the TCP reset problem, servers typically close a connection in stages. First, the server performs a half-close by closing only the write side of the read/write connection. The server then continues to read from the connection until it receives a corresponding close by the client, or until the server is reasonably certain that its own TCP stack has received the client’s acknowledgement of the packet(s) containing the server’s last response. Finally, the server fully closes the connection.

Switching protocols

h11 supports two kinds of “protocol switches”: requests with method CONNECT, and the newer Upgrade: header, most commonly used for negotiating WebSocket connections. Both follow the same pattern: the client proposes that they switch from regular HTTP to some other kind of interaction, and then the server either rejects the suggestion – in which case we return to regular HTTP rules – or else accepts it. (For CONNECT, acceptance means a response with 2xx status code; for Upgrade:, acceptance means an InformationalResponse with status 101 Switching Protocols) If the proposal is accepted, then both sides switch to doing something else with their socket, and h11’s job is done.

As a developer using h11, it’s your responsibility to send and interpret the actual CONNECT or Upgrade: request and response, and to figure out what to do after the handover; it’s h11’s job to understand what’s going on, and help you make the handover smoothly.

Specifically, what h11 does is pause parsing incoming data at the boundary between the two protocols, and then you can retrieve any unprocessed data from the Connection.trailing_data attribute.

Support for sendfile()

Many networking APIs provide some efficient way to send particular data, e.g. asking the operating system to stream files directly off of the disk and into a socket without passing through userspace.

It’s possible to use these APIs together with h11. The basic strategy is:

  • Create some placeholder object representing the special data, that your networking code knows how to “send” by invoking whatever the appropriate underlying APIs are.
  • Make sure your placeholder object implements a __len__ method returning its size in bytes.
  • Call conn.send_with_data_passthrough(Data(data=<your placeholder object>))
  • This returns a list whose contents are a mixture of (a) bytes-like objects, and (b) your placeholder object. You should send them to the network in order.

Here’s a sketch of what this might look like:

class FilePlaceholder:
    def __init__(self, file, offset, count):
        self.file = file
        self.offset = offset
        self.count = count

    def __len__(self):
        return self.count

def send_data(sock, data):
    if isinstance(data, FilePlaceholder):
        # socket.sendfile added in Python 3.5
        sock.sendfile(data.file, data.offset, data.count)
    else:
        # data is a bytes-like object to be sent directly
        sock.sendall(data)

placeholder = FilePlaceholder(open("...", "rb"), 0, 200)
for data in conn.send_with_data_passthrough(Data(data=placeholder)):
    send_data(sock, data)

This works with all the different framing modes (Content-Length, Transfer-Encoding: chunked, etc.) – h11 will add any necessary framing data, update its internal state, and away you go.

Identifying h11 in requests and responses

According to RFC 7231, client requests are supposed to include a User-Agent: header identifying what software they’re using, and servers are supposed to respond with a Server: header doing the same. h11 doesn’t construct these headers for you, but to make it easier for you to construct this header, it provides:

h11.PRODUCT_ID

A string suitable for identifying the current version of h11 in a User-Agent: or Server: header.

The version of h11 that was used to build these docs identified itself as:

In [29]: h11.PRODUCT_ID
Out[29]: 'python-h11/0.14.0'

Chunked Transfer Encoding Delimiters

New in version 0.7.0.

HTTP/1.1 allows for the use of Chunked Transfer Encoding to frame request and response bodies. This form of transfer encoding allows the implementation to provide its body data in the form of length-prefixed “chunks” of data.

RFC 7230 is extremely clear that the breaking points between chunks of data are non-semantic: that is, users should not rely on them or assign any meaning to them. This is particularly important given that RFC 7230 also allows intermediaries such as proxies and caches to change the chunk boundaries as they see fit, or even to remove the chunked transfer encoding entirely.

However, for some applications it is valuable or essential to see the chunk boundaries because the peer implementation has assigned meaning to them. While this is against the specification, if you do really need access to this information h11 makes it available to you in the form of the Data.chunk_start and Data.chunk_end properties of the Data event.

Data.chunk_start is set to True for the first Data event for a given chunk of data. Data.chunk_end is set to True for the last Data event that is emitted for a given chunk of data. h11 guarantees that it will always emit at least one Data event for each chunk of data received from the remote peer, but due to its internal buffering logic it may return more than one. It is possible for a single Data event to have both Data.chunk_start and Data.chunk_end set to True, in which case it will be the only Data event for that chunk of data.

Again, it is strongly encouraged that you avoid relying on this information if at all possible. This functionality should be considered an escape hatch for when there is no alternative but to rely on the information, rather than a general source of data that is worth relying on.

Examples

You can also find these in the examples/ directory of a source checkout.

Minimal client, using synchronous I/O

import socket
import ssl

import h11

################################################################
# Setup
################################################################

conn = h11.Connection(our_role=h11.CLIENT)
ctx = ssl.create_default_context()
sock = ctx.wrap_socket(
    socket.create_connection(("httpbin.org", 443)), server_hostname="httpbin.org"
)

################################################################
# Sending a request
################################################################


def send(event):
    print("Sending event:")
    print(event)
    print()
    # Pass the event through h11's state machine and encoding machinery
    data = conn.send(event)
    # Send the resulting bytes on the wire
    sock.sendall(data)


send(
    h11.Request(
        method="GET",
        target="/get",
        headers=[("Host", "httpbin.org"), ("Connection", "close")],
    )
)
send(h11.EndOfMessage())

################################################################
# Receiving the response
################################################################


def next_event():
    while True:
        # Check if an event is already available
        event = conn.next_event()
        if event is h11.NEED_DATA:
            # Nope, so fetch some data from the socket...
            data = sock.recv(2048)
            # ...and give it to h11 to convert back into events...
            conn.receive_data(data)
            # ...and then loop around to try again.
            continue
        return event


while True:
    event = next_event()
    print("Received event:")
    print(event)
    print()
    if type(event) is h11.EndOfMessage:
        break

################################################################
# Clean up
################################################################

sock.close()

Fairly complete server with error handling, using Trio for async I/O

# A simple HTTP server implemented using h11 and Trio:
#   http://trio.readthedocs.io/en/latest/index.html
#
# All requests get echoed back a JSON document containing information about
# the request.
#
# This is a rather involved example, since it attempts to both be
# fully-HTTP-compliant and also demonstrate error handling.
#
# The main difference between an HTTP client and an HTTP server is that in a
# client, if something goes wrong, you can just throw away that connection and
# make a new one. In a server, you're expected to handle all kinds of garbage
# input and internal errors and recover with grace and dignity. And that's
# what this code does.
#
# I recommend pushing on it to see how it works -- e.g. watch what happens if
# you visit http://localhost:8080 in a webbrowser that supports keep-alive,
# hit reload a few times, and then wait for the keep-alive to time out on the
# server.
#
# Or try using curl to start a chunked upload and then hit control-C in the
# middle of the upload:
#
#    (for CHUNK in $(seq 10); do echo $CHUNK; sleep 1; done) \
#      | curl -T - http://localhost:8080/foo
#
# (Note that curl will send Expect: 100-Continue, too.)
#
# Or, heck, try letting curl complete successfully ;-).

# Some potential improvements, if you wanted to try and extend this to a real
# general-purpose HTTP server (and to give you some hints about the many
# considerations that go into making a robust HTTP server):
#
# - The timeout handling is rather crude -- we impose a flat 10 second timeout
#   on each request (starting from the end of the previous
#   response). Something finer-grained would be better. Also, if a timeout is
#   triggered we unconditionally send a 500 Internal Server Error; it would be
#   better to keep track of whether the timeout is the client's fault, and if
#   so send a 408 Request Timeout.
#
# - The error handling policy here is somewhat crude as well. It handles a lot
#   of cases perfectly, but there are corner cases where the ideal behavior is
#   more debateable. For example, if a client starts uploading a large
#   request, uses 100-Continue, and we send an error response, then we'll shut
#   down the connection immediately (for well-behaved clients) or after
#   spending TIMEOUT seconds reading and discarding their upload (for
#   ill-behaved ones that go on and try to upload their request anyway). And
#   for clients that do this without 100-Continue, we'll send the error
#   response and then shut them down after TIMEOUT seconds. This might or
#   might not be your preferred policy, though -- maybe you want to shut such
#   clients down immediately (even if this risks their not seeing the
#   response), or maybe you're happy to let them continue sending all the data
#   and wasting your bandwidth if this is what it takes to guarantee that they
#   see your error response. Up to you, really.
#
# - Another example of a debateable choice: if a response handler errors out
#   without having done *anything* -- hasn't started responding, hasn't read
#   the request body -- then this connection actually is salvagable, if the
#   server sends an error response + reads and discards the request body. This
#   code sends the error response, but it doesn't try to salvage the
#   connection by reading the request body, it just closes the
#   connection. This is quite possibly the best option, but again this is a
#   policy decision.
#
# - Our error pages always include the exception text. In real life you might
#   want to log the exception but not send that information to the client.
#
# - Our error responses perhaps should include Connection: close when we know
#   we're going to close this connection.
#
# - We don't support the HEAD method, but ought to.
#
# - We should probably do something cleverer with buffering responses and
#   TCP_CORK and suchlike.

import datetime
import email.utils
import json
from itertools import count

import trio

import h11

MAX_RECV = 2**16
TIMEOUT = 10


# We are using email.utils.format_datetime to generate the Date header.
# It may sound weird, but it actually follows the RFC.
# Please see: https://stackoverflow.com/a/59416334/14723771
#
# See also:
# [1] https://www.rfc-editor.org/rfc/rfc9110#section-5.6.7
# [2] https://www.rfc-editor.org/rfc/rfc7231#section-7.1.1.1
# [3] https://www.rfc-editor.org/rfc/rfc5322#section-3.3
def format_date_time(dt=None):
    """Generate a RFC 7231 / RFC 9110 IMF-fixdate string"""
    if dt is None:
        dt = datetime.datetime.now(datetime.timezone.utc)
    return email.utils.format_datetime(dt, usegmt=True)


################################################################
# I/O adapter: h11 <-> trio
################################################################

# The core of this could be factored out to be usable for trio-based clients
# too, as well as servers. But as a simplified pedagogical example we don't
# attempt this here.
class TrioHTTPWrapper:
    _next_id = count()

    def __init__(self, stream):
        self.stream = stream
        self.conn = h11.Connection(h11.SERVER)
        # Our Server: header
        self.ident = " ".join(
            ["h11-example-trio-server/{}".format(h11.__version__), h11.PRODUCT_ID]
        ).encode("ascii")
        # A unique id for this connection, to include in debugging output
        # (useful for understanding what's going on if there are multiple
        # simultaneous clients).
        self._obj_id = next(TrioHTTPWrapper._next_id)

    async def send(self, event):
        # The code below doesn't send ConnectionClosed, so we don't bother
        # handling it here either -- it would require that we do something
        # appropriate when 'data' is None.
        assert type(event) is not h11.ConnectionClosed
        data = self.conn.send(event)
        try:
            await self.stream.send_all(data)
        except BaseException:
            # If send_all raises an exception (especially trio.Cancelled),
            # we have no choice but to give it up.
            self.conn.send_failed()
            raise

    async def _read_from_peer(self):
        if self.conn.they_are_waiting_for_100_continue:
            self.info("Sending 100 Continue")
            go_ahead = h11.InformationalResponse(
                status_code=100, headers=self.basic_headers()
            )
            await self.send(go_ahead)
        try:
            data = await self.stream.receive_some(MAX_RECV)
        except ConnectionError:
            # They've stopped listening. Not much we can do about it here.
            data = b""
        self.conn.receive_data(data)

    async def next_event(self):
        while True:
            event = self.conn.next_event()
            if event is h11.NEED_DATA:
                await self._read_from_peer()
                continue
            return event

    async def shutdown_and_clean_up(self):
        # When this method is called, it's because we definitely want to kill
        # this connection, either as a clean shutdown or because of some kind
        # of error or loss-of-sync bug, and we no longer care if that violates
        # the protocol or not. So we ignore the state of self.conn, and just
        # go ahead and do the shutdown on the socket directly. (If you're
        # implementing a client you might prefer to send ConnectionClosed()
        # and let it raise an exception if that violates the protocol.)
        #
        try:
            await self.stream.send_eof()
        except trio.BrokenResourceError:
            # They're already gone, nothing to do
            return
        # Wait and read for a bit to give them a chance to see that we closed
        # things, but eventually give up and just close the socket.
        # XX FIXME: possibly we should set SO_LINGER to 0 here, so
        # that in the case where the client has ignored our shutdown and
        # declined to initiate the close themselves, we do a violent shutdown
        # (RST) and avoid the TIME_WAIT?
        # it looks like nginx never does this for keepalive timeouts, and only
        # does it for regular timeouts (slow clients I guess?) if explicitly
        # enabled ("Default: reset_timedout_connection off")
        with trio.move_on_after(TIMEOUT):
            try:
                while True:
                    # Attempt to read until EOF
                    got = await self.stream.receive_some(MAX_RECV)
                    if not got:
                        break
            except trio.BrokenResourceError:
                pass
            finally:
                await self.stream.aclose()

    def basic_headers(self):
        # HTTP requires these headers in all responses (client would do
        # something different here)
        return [
            ("Date", format_date_time().encode("ascii")),
            ("Server", self.ident),
        ]

    def info(self, *args):
        # Little debugging method
        print("{}:".format(self._obj_id), *args)


################################################################
# Server main loop
################################################################

# General theory:
#
# If everything goes well:
# - we'll get a Request
# - our response handler will read the request body and send a full response
# - that will either leave us in MUST_CLOSE (if the client doesn't
#   support keepalive) or DONE/DONE (if the client does).
#
# But then there are many, many different ways that things can go wrong
# here. For example:
# - we don't actually get a Request, but rather a ConnectionClosed
# - exception is raised from somewhere (naughty client, broken
#   response handler, whatever)
#   - depending on what went wrong and where, we might or might not be
#     able to send an error response, and the connection might or
#     might not be salvagable after that
# - response handler doesn't fully read the request or doesn't send a
#   full response
#
# But these all have one thing in common: they involve us leaving the
# nice easy path up above. So we can just proceed on the assumption
# that the nice easy thing is what's happening, and whenever something
# goes wrong do our best to get back onto that path, and h11 will keep
# track of how successful we were and raise new errors if things don't work
# out.
async def http_serve(stream):
    wrapper = TrioHTTPWrapper(stream)
    wrapper.info("Got new connection")
    while True:
        assert wrapper.conn.states == {h11.CLIENT: h11.IDLE, h11.SERVER: h11.IDLE}

        try:
            with trio.fail_after(TIMEOUT):
                wrapper.info("Server main loop waiting for request")
                event = await wrapper.next_event()
                wrapper.info("Server main loop got event:", event)
                if type(event) is h11.Request:
                    await send_echo_response(wrapper, event)
        except Exception as exc:
            wrapper.info("Error during response handler: {!r}".format(exc))
            await maybe_send_error_response(wrapper, exc)

        if wrapper.conn.our_state is h11.MUST_CLOSE:
            wrapper.info("connection is not reusable, so shutting down")
            await wrapper.shutdown_and_clean_up()
            return
        else:
            try:
                wrapper.info("trying to re-use connection")
                wrapper.conn.start_next_cycle()
            except h11.ProtocolError:
                states = wrapper.conn.states
                wrapper.info("unexpected state", states, "-- bailing out")
                await maybe_send_error_response(
                    wrapper, RuntimeError("unexpected state {}".format(states))
                )
                await wrapper.shutdown_and_clean_up()
                return


################################################################
# Actual response handlers
################################################################

# Helper function
async def send_simple_response(wrapper, status_code, content_type, body):
    wrapper.info("Sending", status_code, "response with", len(body), "bytes")
    headers = wrapper.basic_headers()
    headers.append(("Content-Type", content_type))
    headers.append(("Content-Length", str(len(body))))
    res = h11.Response(status_code=status_code, headers=headers)
    await wrapper.send(res)
    await wrapper.send(h11.Data(data=body))
    await wrapper.send(h11.EndOfMessage())


async def maybe_send_error_response(wrapper, exc):
    # If we can't send an error, oh well, nothing to be done
    wrapper.info("trying to send error response...")
    if wrapper.conn.our_state not in {h11.IDLE, h11.SEND_RESPONSE}:
        wrapper.info("...but I can't, because our state is", wrapper.conn.our_state)
        return
    try:
        if isinstance(exc, h11.RemoteProtocolError):
            status_code = exc.error_status_hint
        elif isinstance(exc, trio.TooSlowError):
            status_code = 408  # Request Timeout
        else:
            status_code = 500
        body = str(exc).encode("utf-8")
        await send_simple_response(
            wrapper, status_code, "text/plain; charset=utf-8", body
        )
    except Exception as exc:
        wrapper.info("error while sending error response:", exc)


async def send_echo_response(wrapper, request):
    wrapper.info("Preparing echo response")
    if request.method not in {b"GET", b"POST"}:
        # Laziness: we should send a proper 405 Method Not Allowed with the
        # appropriate Accept: header, but we don't.
        raise RuntimeError("unsupported method")
    response_json = {
        "method": request.method.decode("ascii"),
        "target": request.target.decode("ascii"),
        "headers": [
            (name.decode("ascii"), value.decode("ascii"))
            for (name, value) in request.headers
        ],
        "body": "",
    }
    while True:
        event = await wrapper.next_event()
        if type(event) is h11.EndOfMessage:
            break
        assert type(event) is h11.Data
        response_json["body"] += event.data.decode("ascii")
    response_body_unicode = json.dumps(
        response_json, sort_keys=True, indent=4, separators=(",", ": ")
    )
    response_body_bytes = response_body_unicode.encode("utf-8")
    await send_simple_response(
        wrapper, 200, "application/json; charset=utf-8", response_body_bytes
    )


async def serve(port):
    print("listening on http://localhost:{}".format(port))
    try:
        await trio.serve_tcp(http_serve, port)
    except KeyboardInterrupt:
        print("KeyboardInterrupt - shutting down")


################################################################
# Run the server
################################################################

if __name__ == "__main__":
    trio.run(serve, 8080)

Details of our HTTP support for HTTP nerds

h11 only speaks HTTP/1.1. It can talk to HTTP/1.0 clients and servers, but it itself only does HTTP/1.1.

We fully support HTTP/1.1 keep-alive.

We have a little bit of support for HTTP/1.1 pipelining – basically the minimum that’s required by the standard. In server mode we can handle pipelined requests in a serial manner, responding completely to each request before reading the next (and our API is designed to make it easy for servers to keep this straight). Client mode doesn’t support pipelining at all. As far as I can tell, this matches the state of the art in all the major HTTP implementations: the consensus seems to be that HTTP/1.1 pipelining was a nice try but unworkable in practice, and if you really need pipelining to work then instead of trying to fix HTTP/1.1 you should switch to HTTP/2.0.

The HTTP/1.0 Connection: keep-alive pseudo-standard is currently not supported. (Note that this only affects h11 as a server, because h11 as a client always speaks HTTP/1.1.) Supporting this would be possible, but it’s fragile and finicky and I’m suspicious that if we leave it out then no-one will notice or care. HTTP/1.1 is now almost old enough to vote in United States elections. I get that people sometimes write HTTP/1.0 clients because they don’t want to deal with annoying stuff like chunked encoding, and I completely sympathize with that, but I’m guessing that you’re not going to find too many people these days who care desperately about keep-alive and at the same time are too lazy to implement Transfer-Encoding: chunked. Still, this would be my bet as to the missing feature that people are most likely to eventually complain about…

Of the headers defined in RFC 7230, the ones h11 knows and has some special-case logic to care about are: Connection:, Transfer-Encoding:, Content-Length:, Host:, Upgrade:, and Expect: (which is really from RFC 7231 but whatever). The other headers in RFC 7230 are TE:, Trailer:, and Via:; h11 also supports these in the sense that it ignores them and that’s really all it should be doing.

Transfer-Encoding support: we only know chunked, not gzip or deflate. We’re in good company in this: node.js at least doesn’t handle anything besides chunked either. So I’m not too worried about this being a problem in practice. But I’m not majorly opposed to adding support for more features here either.

A quirk in our Response encoding: we don’t bother including ascii status messages – instead of 200 OK we just say 200. This is totally legal and no program should care, and it lets us skip carrying around a pointless table of status message strings, but I suppose it might be worth fixing at some point.

When parsing chunked encoding, we parse but discard “chunk extensions”. This is an extremely obscure feature that allows arbitrary metadata to be interleaved into a chunked transfer stream. This metadata has no standard uses, and proxies are allowed to strip it out. I don’t think anyone will notice this lack, but it could be added if someone really wants it; I just ran out of energy for implementing weirdo features no-one uses.

Currently we do implement support for “obsolete line folding” when reading HTTP headers. This is an optional part of the spec – conforming HTTP/1.1 implementations MUST NOT send continuation lines, and conforming HTTP/1.1 servers MAY send 400 Bad Request responses back at clients who do send them (ref). I’m tempted to remove this support, since it adds some complicated and ugly code right at the center of the request/response parsing loop, and I’m not sure whether anyone actually needs it. Unfortunately a few major implementations that I spot-checked (node.js, go) do still seem to support reading such headers (but not generating them), so it might or might not be obsolete in practice – it’s hard to know.

History of changes

H11 0.14.0 (2022-09-25)

Features
  • Allow additional trailing whitespace in chunk headers for additional compatibility with existing servers. (#133)
  • Improve the type hints for Sentinel types, which should make it easier to type hint h11 usage. (#151 & #144))
Deprecations and Removals
  • Python 3.6 support is removed. h11 now requires Python>=3.7 including PyPy 3. Users running pip install h11 on Python 2 will automatically get the last Python 2-compatible version. (#138)

v0.13.0 (2022-01-19)

Features
  • Clarify that the Headers class is a Sequence and inherit from the collections Sequence abstract base class to also indicate this (and gain the mixin methods). See also #104. (#112)
  • Switch event classes to dataclasses for easier typing and slightly improved performance. (#124)
  • Shorten traceback of protocol errors for easier readability (#132).
  • Add typing including a PEP 561 marker for usage by type checkers (#135).
  • Expand the allowed status codes to [0, 999] from [0, 600] (`#134 https://github.com/python-hyper/h11/issues/134`__).
Backwards incompatible changes

v0.12.0 (2021-01-01)

Features

  • Added support for servers with broken line endings.

    After this change h11 accepts both \r\n and \n as a headers delimiter. (#7)

  • Add early detection of invalid http data when request line starts with binary (#122)

Deprecations and Removals
  • Python 2.7 and PyPy 2 support is removed. h11 now requires Python>=3.6 including PyPy 3. Users running pip install h11 on Python 2 will automatically get the last Python 2-compatible version. (#114)

v0.11.0 (2020-10-05)

New features:

  • h11 now stores and makes available the raw header name as received. In addition h11 will write out header names with the same casing as passed to it. This allows compatibility with systems that expect titlecased header names. See #31.
  • Multiple content length headers are now merged into a single header if all the values are equal, if any are unequal a LocalProtocol error is raised (as before). See #92.

Backwards incompatible changes:

  • Headers added by h11, rather than passed to it, now have titlecased names. Whilst this should help compatibility it replaces the previous lowercased header names.

v0.10.0 (2020-08-14)

Other changes:

  • Drop support for Python 3.4.
  • Support Python 3.8.
  • Make error messages returned by match failures less ambiguous (#98).

v0.9.0 (2019-05-15)

Bug fixes:

  • Allow a broader range of characters in header values. This violates the RFC, but is apparently required for compatibility with real-world code, like Google Analytics cookies (#57, #58).
  • Validate incoming and outgoing request paths for invalid characters. This prevents a variety of potential security issues that have affected other HTTP clients. (#69).
  • Force status codes to be integers, thereby allowing stdlib HTTPStatus IntEnums to be used when constructing responses (#72).

Other changes:

  • Make all sentinel values inspectable by IDEs, and split SEND_BODY_DONE into SEND_BODY, and DONE (#75).
  • Drop support for Python 3.3.
  • LocalProtocolError raised in start_next_cycle now shows states for more informative errors (#80).

v0.8.1 (2018-04-14)

Bug fixes:

  • Always return headers as bytes objects (#60)

Other changes:

  • Added proper license notices to the Javascript used in our documentation (#61)

v0.8.0 (2018-03-20)

Backwards incompatible changes:

  • h11 now performs stricter validation on outgoing header names and header values: illegal characters are now rejected (example: you can’t put a newline into an HTTP header), and header values with leading/trailing whitespace are also rejected (previously h11 would silently discard the whitespace). All these checks were already performed on incoming headers; this just extends that to outgoing headers.

New features:

Bug fixes:

  • Make sure that when computing the framing headers for HEAD responses, we produce the same results as we would for the corresponding GET.
  • Error out if a request has multiple Host: headers.
  • Send the Host: header first, as recommended by RFC 7230.
  • The Expect: header is case-insensitive, so use case-insensitive matching when looking for 100-continue.

Other changes:

  • Better error messages in several cases.
  • Provide correct error_status_hint in exception raised when encountering an invalid Transfer-Encoding header.
  • For better compatibility with broken servers, h11 now tolerates responses where the reason phrase is missing (not just empty).
  • Various optimizations and documentation improvements.

v0.7.0 (2016-11-25)

New features (backwards compatible):

Bug fixes:

  • Fix the error message given when a call to an event constructor is missing a required keyword argument (issue #14).
  • Fixed encoding of empty Data events (Data(data=b"")) when using chunked encoding (issue #21).

v0.6.0 (2016-10-24)

This is the first release since we started using h11 to write non-trivial server code, and this experience triggered a number of substantial API changes.

Backwards incompatible changes:

  • Split the old receive_data() into the new receive_data() and next_event(), and replaced the old Paused pseudo-event with the new NEED_DATA and PAUSED sentinels.
  • Simplified the API by replacing the old Connection.state_of(), Connection.client_state, Connection.server_state with the new Connection.states.
  • Renamed the old prepare_to_reuse() to the new start_next_cycle().
  • Removed the Paused pseudo-event.

Backwards compatible changes:

  • State machine: added a DONE -> MUST_CLOSE transition triggered by our peer being in the ERROR state.
  • Split ProtocolError into LocalProtocolError and RemoteProtocolError (see Error handling). Use case: HTTP servers want to be able to distinguish between an error that originates locally (which produce a 500 status code) versus errors caused by remote misbehavior (which produce a 4xx status code).
  • Changed the PRODUCT_ID from h11/<verson> to python-h11/<version>. (This is similar to what requests uses, and much more searchable than plain h11.)

Other changes:

  • Added a minimal benchmark suite, and used it to make a few small optimizations (maybe ~20% speedup?).

v0.5.0 (2016-05-14)

  • Initial release.