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pyfakefs implements a fake file system that mocks the Python file system modules.

Project Description

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:


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 unittest.mock

The other approach is to do the patching yourself using mock.patch():

import pyfakefs.fake_filesystem as fake_fs

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

# Do some setup on the faked file system

# 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

import pyfakefs.fake_filesystem_glob as fake_glob

# Note that this fake module is based on the fake fs you just created
glob = fake_glob.FakeGlobModule(fs)
with patch('mymodule.glob', glob):



pyfakefs works with Python 2.6 and above, on Linux, Windows and OSX (MacOS).

pyfakefs requires mox3.

pyfakefs works with PyTest version 2.8.0 or above.


Continuous integration

pyfakefs is automatically tested with Python 2.6 and above, and it is currently .

See Travis-CI for test results for each Python version.

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

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: * 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 maintains and extends pyfakefs.

Release History

This version
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