Python Notes¶
Table of Contents¶
About Python¶
What’s Python ?
- Python is an interpreted language, it is not a compiled language. This means that it needs to be run by another program, called the interpreter (/usr/bin/python in Linux or python.exe in Window) to generate the bytecode
*.pyc, rather than the processor itself. Not like as C, which runs directly on the processor, after a compilation to bytecode. - Because of interpreted languages, Python is a high-level programming language. For exemple, an important feature of high-level programming language is garbage collector, which free automatiquement the variable on memory at the end of program or function, to avoid memory leak.
- In France, in 2017, python becomes the best programmation language (after this french site )
Compare Python and other language
This discussion is very huge on the internet, and there’s some notes :
| Feature | Python
|
C
|
|---|---|---|
| Pointer | In python we dont have the definition
of pointer. All variables are names
bound to objects ! We will speak it at
topic pointer .
|
Yes
|
| Type | Dynamic ! we can change object type
type of variables at run-time
|
Static typing !
|
| Varaible location | All objects in python stock in heap
(whatever
int, float...). Onlythe name of variable sits in stack
|
We can define where we
want to
stock via pointer
|
| Memory & Performance | Very inefficient !
Example, a list [1,2,3] take 200 bytes.
Slowly !
|
Very efficient !
Ex, a list {1,2,3} take
only 16 bytes
Fast !
|
Python 2.7
- This version is the most stable release, and the most frequently used. This is scheduled to be the last major version in the 2.x series before it moves into an extended maintenance period. For me, I always work with version 2.7, so all of my notes in this site go with this version.
- We must know that in 2020, python 2.7 will be not supported anymore and all modules will be released only for python3 :( Sometimes I try it, but the syntax is very different from version 2 but we have no choice, so now this’s time to move to python 3 ! I advise that if we start a project now, we should try & work with version 3 with their packages/modules, and our project will be safe in 2020 :)
- To show the current python using, on linux, we do
$ ls -l /usr/bin/python
/usr/bin/python -> python2.7
Install Python & Package¶
Install Python¶
Linux
Gererally, python is installed by default in Linux OS at the path /usr/bin/python. To find this path, there are many ways, for me I like :
$ which python
/usr/bin/python
Window
To isntall python, we do these steps:
Download python installation at this (site). Select the version you need to.
Double click on the downloaded file, then click next next ... to install. You can choose the specific directory also. For me I put it as default directory D:Python27
To use easily, we should create a python environment variable on window :
- Open Control Panel, then System
- Click ‘Advanced system settings’ on the left
- Click the ‘Environment Variables’ button
- Under ‘System variables’ click the variable called ‘Path’ then the ‘Edit...’ button. (This will set it for all users, you could instead choose to edit the User variables to just set python as a command prompt command for the current user)
- Without deleting any other text, add D:Python27; (include the semi-colon) to the beginning of the ‘Variable value’ and click OK.
- Click OK on the ‘Environment Variables’ window.
Install Module Python¶
- As a popular open source development project, Python has an active supporting community of contributors and users that also make their software available for other Python developers to use under open source license terms.
- This allows Python users to share and collaborate effectively, benefiting from the solutions others have already created to common (and sometimes even rare!) problems, as well as potentially contributing their own solutions to the common pool.
- pip is the preferred installer program. Starting with Python 3.4, it is included by default with the Python binary installers.
Pip Installation
Linux
To install python
$ sudo apt-get install python-pip python-dev build-essential
$ sudo pip install --upgrade pip
$ sudo pip install --upgrade virtualenv
Window
With the current python vesion, pip is located at <path_to_python_dir>/Python27/Scripts/ We see pip.exe : that is pip application. So to use easily, we should create a environement variable. Unless, each time we want to install a package, navigating to pip directory, and typing the pip commands.
Basic Usage
To install the package, you can use the following command :
Search a package :
$pip search package_name
Install :
$pip install package_name
Uninstall :
$pip uninstall package_name
Run Python Script¶
Run Python¶
To run a script in python, just type python script.py , then our program will compile and run at the same time
Note
In fact, python use a program, called the interpreter (/usr/bin/python in Linux or python.exe in Window) to compile our source code to the bytecode *.pyc, then execute this bytecode.
Python Interactive¶
The Python interpreter is usually installed at the path of python installation. To open this, juste typing python to the shell, we have :
$ python
Python 2.7.9 (default, Mar 1 2015, 12:57:24)
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
and from now, we can apply and test all python commands here !!
Tip
For me, this tool is very important on my work. When I write a new function, always I test it directly on Interactive Console with maximum testcase possible. This way helps me avoid many stupid mistakes.
Python command options¶
We mostly use python -m mymodule to run a python source code . But there are other common command-lines options :
python [-c command | -m module-name | script | - ] [args]
-c
The -c cmd option can be used to execute short programs in the form of a command-line option—for example:
$ python -c "print('hello world')".
hello world
-m
Runs a library module as a script which executes inside the __main__ module prior to the execution of the main script.
For this command, example python -m mymodule : Python does 2 things :
- First, import the packages
mymodule. If the given module isn’t located on the Python module path, an error is handled here and the program will be stop.- Second, run this module
mymodulelike as a script.
Exemple : I have a script foo.py
print 'hello'
print __name__
then we run this script by 2 ways:
$ python foo.py
hello
__main__
$ python -m foo
hello
We have the same result ! Attention with the path to our module, it raise an error if the module isn’t in the PYTHON_PATH.We shall see it at sys module
Tip
Programs must be written for people to read, and only incidentally for machines to execute. — Abelson & Sussman, Structure and Interpretation of Computer Programs
Python Code Style¶
Text Editor
I recommend these text editors for python development :
- Sublime text : very beautiful interface, Python syntax highlighting, Python plugins.
- vim : for all linuxer
- NotePad++ : I always use this editor although my friends mocking me :(( Having a perfect NppFPT for virtual machine, and mostly it has an option to backup all my source code each time I do Ctrl+S.
- Pycharm : Full-featured IDE for Python. I tried it once, a very nice interface, autocorrect following PEP8 standart, and efficient but it’s so slow.
Indentation¶
Whitespace 1
- 4 spaces per indentation level.
- Never mix tabs and spaces.
- One blank line between functions.
- Two blank lines between classes.
Whitespace 2
- Add a space after ”,” in
dict,list,tuple, & argument lists, and after ”:” in dicts, but not before.- Put spaces around assignments & comparisons (except in argument lists).
- No spaces just inside parentheses or just before argument lists.
Exemple:
def make_squares(key, value=0):
"""
Return a dictionary and a list.
@param
key : string or numeric
value : any type, by default is 0
@return
a tuple 2 element, the first is dictionnary, other is list
- Exemple :
make_squares(4)
make_squares(4,99)
"""
d = {key: value}
l = [key, value]
return d, l
Convention in Python for variable and function names¶
- Class: ClassName
- Method name : method_name
- Function : names should be lowercase, with words separated by underscores as necessary to improve readability , example
function_name. - Variables : lowercase with words separated by underscores as necessary to improve readability.
- Private methods and properties start with __double_underscore
- “Protected” methods and properties start with _single_underscore
- global_var_name
- instance_var_name
- local_var_name
- function_parameter_name
- Constant name : GLOBAL_CONSTANT_NAME
- ExceptionName : ExceptionName
Ignored variable
If you need to assign something but will not need that variable, use the double underscores __ (not a single underscore _ in order to avoid confusion with variable to store the result of the last evaluation) :
filename = 'foobar.txt'
basename, __, ext = filename.rpartition('.')
Docstrings & Comments¶
Docstrings : Explain how to use code, and are for the users of our code. This is written between 2 triple-quotes. This must always have 3 things :
- Purpose of the function
- Description the given parameters (name, type, note), we use @param ; the return values (name, type, note), we use @return.
- Un example to run this function
A linebreak after a block. Exemple :
def sum3(a,b,c) :
"""
This function to get the sum of 3 given numbers.
@param:
a, b, c : numeric type, raise exception if it lacks one
@return:
my_sum : numeric type
- Example : sum3(3, 4.4, -1)
"""
return a + b + c
Note
When the function is called, the Docstrings is in method __doc__. For the above example, typing print sum3.__doc__ or help(sum3) in python interactive and it show our docstring.
Comments : Explain why, and are for the maintainers of our code. Genarally there are 3 types :
- Block Comments
- Inline Comments
- Commenting Out Code for Testing
autopep8¶
The library autopep8 automatically formats Python code to conform to the PEP 8 style guide. So good ! For example, I have a python script named my_script.py was bad written, by using this lib, we are safe !
pip install autopep8
autopep8 --in-place my_script.py
if __name__ == “__main__”¶
Sometimes we see this notion in source code, that means if we run directly the script from terminal, these command-lines in if block will be executed .By example we have a script a.py :
if __name__ == "__main__":
print 'hello'
Then run in cmd:
>>> python a.py
hello
But if we import a into another script python, all commands in if __name__ == "__main__" will be not execute, because in this case, __name__ become ‘a’. Exemple we have the script a.py like as above, then we import a.py into b.py:
import a
if __name__ == "__main__":
print 'hello b'
print a.__name__
we run :
>>> python b.py
hello b
a
What’s the use ?
This thing’s used for testing when we write a new module or new sub-script in a grand project. For my above exemple, I can write some testsuite after if __name__ == “__main__”:
Operating_System_Modules¶
This section speak about 2 important modules in Python, os and sys.
Basic¶
Tips¶
Variable Types¶
Built-In Types : 12
Basic¶
Mutable Object
Pointer¶
>>> i = 5
>>> j = i
>>> j = 3
>>> print(i)
5
We see that i refers to an integer on memory has value 5 at first line, then j refers to i, means j also refers to 5. But when we change j =3, that means j points to another location on memory. Because i is an integer which is an immutable object, so there’is not any change on i. And whats about mutable object list ?
>>> a = [0, 1, 2, 3, 4]
>>> b = a
>>> b[2] = 9999
>>> a
[0, 1, 9999, 3, 4]
If 2 lists a and b refer at same object, when a changes, b changes also !