pyfakefs

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 creates a new empty in-memory file system at each test start, which replaces
the real filesystem during the test. Think of pyfakefs as making a per-test temporary
directory, except for an entire file system.

There are several means to achieve this: by using
the fs fixture if running pytest, by using fake_filesystem_unittest.TestCase as a
base class if using unittest, by using a fake_filesystem_unittest.Patcher instance
as a context manager, or by using the patchfs decorator.

pyfakefs works with current versions of Linux, Windows and macOS.

Documentation

This document provides a general overview for pyfakefs. There is more:

The documentation at Read the Docs:
- The Release documentation
  contains usage documentation for pyfakefs and a description of the
  most relevant classes, methods and functions for the last version
  released on PyPI
- The Development documentation
  contains the same documentation for the current main branch
- The Release 3.7 documentation
  contains usage documentation for the last version of pyfakefs
  supporting Python 2.7
The Release Notes
show a list of changes in the latest versions

Usage

The simplest method to use pyfakefs is using the fs fixture with pytest.
Refer to the
usage documentation
for information on other test scenarios, test customization and
using convenience functions.

Features

Apart from automatically mocking most file-system functions, pyfakefs
provides some additional features:

mapping files and directories from the real file system into the fake filesystem
configuration and tracking of the file system size
pause and resume of patching to be able to use the real file system inside a
test step
(limited) emulation of other OSes (Linux, macOS or Windows)
configuration to behave as if running as a non-root user while running
under root

Compatibility

pyfakefs works with CPython 3.7 and above, on Linux, Windows and macOS, and
with PyPy3.

pyfakefs works with pytest version 6.2.5 or above,
though a current version is recommended.

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. Refer to the
documentation
for more information about the limitations of pyfakefs.

Development

Continuous integration

pyfakefs is currently automatically tested on Linux, macOS and Windows, with
Python 3.7 to 3.13, and with PyPy3 on Linux, using
GitHub Actions.

Running pyfakefs unit tests

On the command line

pyfakefs unit tests can be run using pytest (all tests) or unittest
(all tests except pytest-specific ones):

$ cd pyfakefs/
$ export PYTHONPATH=$PWD

$ python -m pytest pyfakefs
$ python -m pyfakefs.tests.all_tests

Similar scripts are called by tox and Github Actions. tox can be used to
run tests locally against supported python versions:

$ tox

In a Docker container

The Dockerfile at the repository root 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.

History

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 was used in over 20,000
Python tests at Google.

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

Fork
jmcgeheeiv-pyfakefs added
direct support for unittest and doctest
Fork
shiffdane-jmcgeheeiv-pyfakefs
added further corrections

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. In 2022, the repository has been transferred to
pytest-dev to ensure continuous maintenance.

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pyfakefs

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