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Overview

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Declaring singleton classes and singleton factories with different scopes of instantiation, striving for thread-safety and simplicity.

  • Free software: MIT license

Installation

pip install singletons

Quick Example

import singletons

@singletons.GlobalFactory
def my_uuid():
    return uuid.uuid4()

# elsewhere...
my_uuid()  # will return the global instance of a UUID object

Development

To run the all tests run:

tox

Usage

One of the simplest ways to use singletons is using a factory decorator to make the return value of a function a singleton object. Create a shared.py file:

import uuid
import singletons

@singletons.GlobalFactory
def my_uuid():
    return uuid.uuid4()

Any time you want to access the instance generated by your factory, just call the my_uuid() function.

Factory decorators include:

  • GlobalFactory
  • ProcessFactory
  • ThreadFactory
  • GreenthreadFactory
  • EventletFactory
  • GeventFactory

You can also declare a class as a singleton by using the metaclass keyword argument:

import singletons

class SharedCache(dict, metaclass=singletons.ThreadSingleton):
    pass

When SharedCache is called (using SharedCache()), if an object already exists for the current thread it is returned, otherwise it is constructed.

Singleton metaclasses include:

  • Singleton
  • ProcessSingleton
  • ThreadSingleton
  • GreenthreadSingleton
  • EventletSingleton
  • GeventSingleton

Writing Tests

A common need when working with singletons is to be able to use Mock objects for unit tests. singletons includes a helper class for making modules easily swappable to use Mocks for everything instead of the factories/classes defined. A common usage would be to put these lines at the bottom of your shared.py file:

class _Shared(singletons.SharedModule):
    globals = globals()
sys.modules[__name__] = _Shared()

To enable the Mock object replacement, call setup_mock() or set the environment variable SINGLETONS_SETUP_MOCK=1. This will replace all accesses of module attributes with Mock() instances. setup_mock can be called inside a TestCase setup() method or as part of a pytest fixture to ensure that each test has a clean set of Mock() instances.

Example test:

class MyTestCase(unittest.TestCase):
    def setup(self):
        shared.setup_mock()

    def test_get_documents():
        c = shared.session()
        # do thing
        c.request.assert_called_once()

To use custom Mock objects, set them as attributes on the module after calling setup_mock:

class MyTestCase(unittest.TestCase):
    def setup(self):
        shared.setup_mock()
        mock_instance = mock.Mock(spec=User)
        mock_instance.name = 'Jane Doe'
        mock_instance.username = 'jdoe123'
        shared.mock_instance = mock_instance

    def test_get_userdata():
        c = shared.mock_instance()
        # do thing
        c.request.assert_called_once()

Reference

singletons

Contributing

Contributions are welcome, and they are greatly appreciated! Every little bit helps, and credit will always be given.

Bug reports

When reporting a bug please include:

  • Your operating system name and version.
  • Any details about your local setup that might be helpful in troubleshooting.
  • Detailed steps to reproduce the bug.

Documentation improvements

singletons could always use more documentation, whether as part of the official singletons docs, in docstrings, or even on the web in blog posts, articles, and such.

Feature requests and feedback

The best way to send feedback is to file an issue at https://github.com/jmaroeder/python-singletons/issues.

If you are proposing a feature:

  • Explain in detail how it would work.
  • Keep the scope as narrow as possible, to make it easier to implement.
  • Remember that this is a volunteer-driven project, and that code contributions are welcome :)

Development

To set up python-singletons for local development:

  1. Fork python-singletons (look for the “Fork” button).

  2. Clone your fork locally:

    git clone git@github.com:your_name_here/python-singletons.git

  3. Create a branch for local development:

    git checkout -b name-of-your-bugfix-or-feature

    Now you can make your changes locally.

  4. When you’re done making changes, run all the checks, doc builder and spell checker with tox one command:

    tox

  5. Commit your changes and push your branch to GitHub:

    git add .
git commit -m "Your detailed description of your changes."
git push origin name-of-your-bugfix-or-feature

  6. Submit a pull request through the GitHub website.

Pull Request Guidelines

If you need some code review or feedback while you’re developing the code just make the pull request.

For merging, you should:

  1. Include passing tests (run tox) [1].
  2. Update documentation when there’s new API, functionality etc.
  3. Add a note to CHANGELOG.rst about the changes.
  4. Add yourself to AUTHORS.rst.
[1]

If you don’t have all the necessary python versions available locally you can rely on Travis - it will run the tests for each change you add in the pull request.

It will be slower though …

Tips

To run a subset of tests:

tox -e envname -- py.test -k test_myfeature

To run all the test environments in parallel (you need to pip install detox):

detox

Authors

Changelog

master

0.2.2 (2018-02-01)

  • Shows warning rather than giving exception when using greenthread singletons without a greenthread environment

0.2.1 (2018-01-29)

  • CI changes

0.2.0 (2018-01-23)

  • More tests
  • Usage examples added to documentation
  • Fixed issues revealed by tests

0.1.0 (2018-01-22)

  • First release on PyPI.

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