pyfakefs implements a fake file system that mocks the Python file system modules.
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Latest commit ee5bb58 Feb 22, 2018
cclauss and mrbean-bremen Add flake8 to testing
* FileNotFoundError, FileExistsError, NotADirectoryError exist in Python 3 but not in Python 2.
* file(), long, unicode exist in Python 2 but not in Python 3


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 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 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 This URL always points to the most relevant top page for pyfakefs.


There are several approaches to implementing tests using pyfakefs.

Automatically find and patch

The first approach is to allow pyfakefs to automatically find all real file functions and modules, and stub these out with the fake file system functions and modules.
This is explained in the pyfakefs wiki page Automatically find and patch file functions and modules and demonstrated in files and

Patch using the PyTest plugin

If you use PyTest, you will be interested in the PyTest plugin in pyfakefs. This automatically patches all file system functions and modules in a manner similar to the automatic find and patch approach described above.

The PyTest plugin provides the fs fixture for use in your test. For example:

def my_fakefs_test(fs):
    # "fs" is the reference to the fake file system
    assert os.path.exists('/var/data/xx1.txt')

Patch using fake_filesystem_unittest.Patcher

If you are using other means of testing like nose, you can do the patching using fake_filesystem_unittest.Patcher - the class doing the the actual work of replacing the filesystem modules with the fake modules in the first two approaches.

The easiest way is to just use Patcher as a context manager:

from fake_filesystem_unittest import Patcher

with Patcher() as patcher:
   # access the fake_filesystem object via patcher.fs
   patcher.fs.create_file('/foo/bar', contents='test')

   # the following code works on the fake filesystem
   with open('/foo/bar') as f:
       contents =

You can also initialize Patcher manually:

from fake_filesystem_unittest import Patcher

patcher = Patcher()
patcher.setUp()     # called in the initialization code
patcher.tearDown()  # somewhere in the cleanup code

Patch using unittest.mock (deprecated)

You can also use mock.patch() to patch the modules manually. This approach will only work for the directly imported modules, therefore it is not suited for testing larger code bases. As the other approaches are more convenient, this one is considered deprecated. You have to create a fake filesystem object, and afterwards fake modules based on this file system for the modules you want to patch.

The following modules and functions can be patched:

  • os and os.path by fake_filessystem.FakeOsModule
  • io by fake_filessystem.FakeIoModule
  • pathlib by fake_pathlib.FakePathlibModule
  • build-in open() by fake_filessystem.FakeFileOpen
   import pyfakefs.fake_filesystem as fake_fs

   # Create a faked file system
   fs = fake_fs.FakeFilesystem()

   # Do some setup on the faked file system
   fs.CreateFile('/foo/bar', contents='test')

   # Replace some built-in file system related modules you use with faked ones

   # Assuming you are using the mock library to ... mock things
       from unittest.mock import patch  # In Python 3, mock is built-in
   except ImportError:
       from mock import patch  # Python 2

   # Note that this fake module is based on the fake fs you just created
   os = fake_fs.FakeOsModule(fs)
   with patch('mymodule.os', os):
       fd ='/foo/bar', os.O_RDONLY)
       contents =, 4)



pyfakefs works with CPython 2.7, 3.3 and above, on Linux, Windows and OSX (MacOS), and with PyPy2 and 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.


pyfakefs is available on PyPi.


Continuous integration

pyfakefs is currently automatically tested:

  • On Linux, with Python 2.7, 3.3 and above using Travis
  • On MacOS, with Python 2.7 and 3.6, also using Travis. The Linux/MacOS build is currently Build Status.
  • On Windows, with Python 2.7, 3.4 and above using Appveyor. The Windows build is currently Build status.

Running pyfakefs unit tests

pyfakefs unit tests are available via two test scripts:

$ python
$ py.test

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

$ tox

Contributing to pyfakefs

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

History 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.