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pyfakefs PyPI version Python version

pyfakefs implements a fake file system that mocks the Python file system modules. Using pyfakefs, your tests operate on a fake file system in memory without touching the real disk. The software under test requires no modification to work with pyfakefs.

pyfakefs works with Linux, Windows and MacOS.


This file provides general usage instructions for pyfakefs. There is more:

  • The documentation at GitHub Pages:
    • The Release documentation contains usage documentation for pyfakefs and a description of the most relevent classes, methods and functions for the last version released on PyPi
    • The Development documentation contains the same documentation for the current master branch
    • The Release 3.7 documentation contains usage documentation for the last version of pyfakefs supporting Python 2.7
    • The Release 3.3 documentation contains usage documentation for the last version of pyfakefs supporting Python 2.6, and for the old-style API (which is still supported but not documented in the current release)
  • The Release Notes show a list of changes in the latest versions

Linking to pyfakefs

In your own documentation, please link to pyfakefs using the canonical URL http://pyfakefs.org. This URL always points to the most relevant top page for pyfakefs.


pyfakefs has support for unittest and pytest, but can also be used directly using fake_filesystem_unittest.Patcher. Refer to the usage documentation for more information on test scenarios, test customization and using convenience functions.


pyfakefs works with CPython 3.5 and above, on Linux, Windows and OSX (MacOS), and with PyPy3.

pyfakefs works with PyTest version 2.8.6 or above.

pyfakefs will not work with Python libraries that use C libraries to access the file system. This is because pyfakefs cannot patch the underlying C libraries' file access functions--the C libraries will always access the real file system. For example, pyfakefs will not work with lxml. In this case lxml must be replaced with a pure Python alternative such as xml.etree.ElementTree.


Continuous integration

pyfakefs is currently automatically tested:

  • Build Status on Linux, with Python 3.5 to 3.8, using Travis
  • Build Status on MacOS, with Python 3.6 to 3.8, using Travis
  • Build status on Windows, with Python 3.5 to 3.8 using Appveyor

Running pyfakefs unit tests

On the command line

pyfakefs unit tests can be run using unittest or pytest:

$ cd pyfakefs/

$ python -m pyfakefs.tests.all_tests
$ python -m pyfakefs.tests.all_tests_without_extra_packages
$ python -m pytest pyfakefs/pytest_tests/pytest_plugin_test.py

These scripts are called by tox and Travis-CI. tox can be used to run tests locally against supported python versions:

$ tox

In a Docker container

The Dockerfile at the top of the repository will run the tests on the latest Ubuntu version. Build the container:

cd pyfakefs/
docker build -t pyfakefs .

Run the unit tests in the container:

docker run -t pyfakefs

Contributing to pyfakefs

We always welcome contributions to the library. Check out the Contributing Guide for more information.


pyfakefs.py was initially developed at Google by Mike Bland as a modest fake implementation of core Python modules. It was introduced to all of Google in September 2006. Since then, it has been enhanced to extend its functionality and usefulness. At last count, pyfakefs is used in over 2,000 Python tests at Google.

Google released pyfakefs to the public in 2011 as Google Code project pyfakefs:

After the shutdown of Google Code was announced, John McGehee merged all three Google Code projects together here on GitHub where an enthusiastic community actively supports, maintains and extends pyfakefs.