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 CPython 3.7 and above, on Linux, Windows and OSX (MacOS), and with PyPy3.

pyfakefs works with pytest version 3.0.0 or above.


pyfakefs is available on PyPi. The latest released version can be installed from pypi:

pip install pyfakefs

The latest master can be installed from the GitHub sources:

pip install git+


  • Code executed under pyfakefs works transparently on a memory-based file system without the need of special commands. The same code that works on the real filesystem will work on the fake filesystem if running under pyfakefs.
  • pyfakefs provides direct support for unittest (via a TestCase base class) and pytest (via a fixture), but can also be used with other test frameworks.
  • Each pyfakefs test starts with an empty file system, but it is possible to map files and directories from the real file system into the fake filesystem if needed.
  • No files in the real file system are changed during the tests, even in the case of writing to mapped real files.
  • pyfakefs keeps track of the filesystem size if configured. The file system size can be configured arbitrarily.
  • it is possible to pause and resume using the fake filesystem, if the real file system has to be used in a test step
  • pyfakefs defaults to the OS it is running on, but can also be configured to test code running under another OS (Linux, MacOS or Windows).
  • pyfakefs can be configured to behave as if running as a root or as a non-root user, independently from the actual user.


  • pyfakefs will not work with Python libraries (other than os and io) that use C libraries to access the file system, because it cannot patch the underlying C libraries’ file access functions
  • pyfakefs patches most kinds of importing file system modules automatically, but there are still some cases where this will not work. See Customizing patching for more information and ways to work around this.
  • pyfakefs does not retain the MRO for file objects, so you cannot rely on checks using isinstance for these objects (for example, to differentiate between binary and textual file objects).
  • pyfakefs is only tested with CPython and the newest PyPy versions, other Python implementations will probably not work
  • Differences in the behavior in different Linux distributions or different MacOS or Windows versions may not be reflected in the implementation, as well as some OS-specific low-level file system behavior. The systems used for automatic tests in Travis.CI and AppVeyor are considered as reference systems, additionally the tests are run in Docker containers with the latest CentOS, Debian, Fedora and Ubuntu images.
  • pyfakefs may not work correctly if file system functions are patched by other means (e.g. using unittest.mock.patch) - see Pyfakefs and mock_open for more information
  • pyfakefs will not work correctly with behave due to the way it loads the steps, if any filesystem modules are imported globally in the steps or environment files; as a workaround, you may load them locally inside the test steps (see this issue)


pyfakefs 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 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 on GitHub where an enthusiastic community actively maintains and extends pyfakefs.