Test Scenarios

There are several approaches to implementing tests using pyfakefs.

Patch using fake_filesystem_unittest

If you are using the Python unittest package, the easiest approach is to use test classes derived from fake_filesystem_unittest.TestCase.

If you call setUpPyfakefs() in your setUp(), pyfakefs will automatically find all real file functions and modules, and stub these out with the fake file system functions and modules:

from pyfakefs.fake_filesystem_unittest import TestCase

class ExampleTestCase(TestCase):
    def setUp(self):

    def test_create_file(self):
        file_path = '/test/file.txt'

The usage is explained in more detail in Automatically find and patch file functions and modules and demonstrated in the 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 similar manner as 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 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 pyfakefs.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 pyfakefs.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 and will not be described in detail.

Customizing Patcher and TestCase

Both fake_filesystem_unittest.Patcher and fake_filesystem_unittest.TestCase provide a few arguments to handle cases where patching does not work out of the box. In case of fake_filesystem_unittest.TestCase, these arguments can either be set in the TestCase instance initialization, or passed to setUpPyfakefs().


If you need these arguments in PyTest, you must use Patcher directly instead of the fs fixture. Alternatively, you can add your own fixture with the needed parameters.

An example for both approaches can be found in with the example fixture in We advice to use this example fixture code as a template for your customized pytest plugins.


Pyfakefs patches modules that are imported before starting the test by finding and replacing file system modules in all loaded modules at test initialization time. This allows to automatically patch file system related modules that are:

  • imported directly, for example:
import os
import pathlib.Path
  • imported as another name:
import os as my_os
  • imported using one of these two specially handled statements:
from os import path
from pathlib import Path

Additionally, functions from file system related modules are patched automatically if imported like:

from os.path import exists
from os import stat

This also works if importing the functions as another name:

from os.path import exists as my_exists
from io import open as io_open
from builtins import open as bltn_open

There are a few cases where automatic patching does not work. We know of two specific cases where this is the case:

  • initializing global variables:
from pathlib import Path

path = Path("/example_home")

In this case, path will hold the real file system path inside the test.

  • initializing a default argument:
import os

def check_if_exists(filepath, file_exists=os.path.exists):
    return file_exists(filepath)

Here, file_exists will not be patched in the test.

To get these cases to work as expected under test, the respective modules containing the code shall be added to the modules_to_reload argument (a module list). The passed modules will be reloaded, thus allowing pyfakefs to patch them dynamically. All modules loaded after the initial patching described above will be patched using this second mechanism.

Given that the example code shown above is located in the file example/, the following code will work:

# example using unittest
class ReloadModuleTest(fake_filesystem_unittest.TestCase):
    def setUp(self):

    def test_path_exists(self):
        file_path = '/foo/bar'

# example using Patcher
def test_path_exists():
    with Patcher() as patcher:
      file_path = '/foo/bar'
      assert example.sut.check_if_exists(file_path)

Example using pytest:

from example import sut

def fs_reload_sut():
    patcher = Patcher(modules_to_reload=[sut])
    patcher.setUp() = patcher.original_open
    tokenize._builtin_open = patcher.original_open
    yield patcher.fs

def test_path_exists(fs_reload_sut):
    file_path = '/foo/bar'
    assert example.sut.check_if_exists(file_path)


Sometimes there are file system modules in other packages that are not patched in standard pyfakefs. To allow patching such modules, modules_to_patch can be used by adding a fake module implementation for a module name. The argument is a dictionary of fake modules mapped to the names to be faked.

This mechanism is used in pyfakefs itself to patch the external modules pathlib2 and scandir if present, and the following example shows how to fake a module in Django that uses OS file system functions:

class FakeLocks(object):
    """django.core.files.locks uses low level OS functions, fake it."""
    _locks_module = django.core.files.locks

    def __init__(self, fs):
        """Each fake module expects the fake file system as an __init__
        # fs represents the fake filesystem; for a real example, it can be
        # saved here and used in the implementation

    def lock(f, flags):
        return True

    def unlock(f):
        return True

    def __getattr__(self, name):
        return getattr(self._locks_module, name)

# test code using Patcher
with Patcher(modules_to_patch={'django.core.files.locks': FakeLocks}):

# test code using unittest
class TestUsingDjango(fake_filesystem_unittest.TestCase):
    def setUp(self):
        self.setUpPyfakefs(modules_to_patch={'django.core.files.locks': FakeLocks})

    def test_django_stuff()


This may be used to add modules that shall not be patched. This is mostly used to avoid patching the Python file system modules themselves, but may be helpful in some special situations, for example if a testrunner is accessing the file system after test setup. A known case is erratic behavior if running a debug session in PyCharm with Python 2.7, which can be avoided by adding the offending module to additional_skip_names:

with Patcher(additional_skip_names=['pydevd']) as patcher:

Alternatively to the module names, the modules themselves may be used:

import pydevd

with Patcher(additional_skip_names=[pydevd]) as patcher:

There is also the global variable Patcher.SKIPNAMES that can be extended for that purpose, though this seldom shall be needed (except for own pytest plugins, as shown in the example mentioned above).


This is True by default, meaning that the user is considered a root user if the real user is a root user (e.g. has the user ID 0). If you want to run your tests as a non-root user regardless of the actual user rights, you may want to set this to False.

Using convenience methods

While pyfakefs can be used just with the standard Python file system functions, there are few convenience methods in fake_filesystem that can help you setting up your tests. The methods can be accessed via the fake_filesystem instance in your tests: Patcher.fs, the fs fixture in PyTest, or TestCase.fs.

File creation helpers

To create files, directories or symlinks together with all the directories in the path, you may use create_file(), create_dir() and create_symlink(), respectively.

create_file() also allows you to set the file mode and the file contents together with the encoding if needed. Alternatively, you can define a file size without contents - in this case, you will not be able to perform standard IO operations on the file (may be used to “fill up” the file system with large files).

from pyfakefs.fake_filesystem_unittest import TestCase

class ExampleTestCase(TestCase):
    def setUp(self):

    def test_create_file(self):
        file_path = '/foo/bar/test.txt'
        self.fs.create_file(file_path, contents = 'test')
        with open(file_path) as f:

create_dir() behaves like os.makedirs(), but can also be used in Python 2.

Access to files in the real file system

If you want to have read access to real files or directories, you can map them into the fake file system using add_real_file(), add_real_directory(), add_real_symlink() and add_real_paths(). They take a file path, a directory path, a symlink path, or a list of paths, respectively, and make them accessible from the fake file system. By default, the contents of the mapped files and directories are read only on demand, so that mapping them is relatively cheap. The access to the files is by default read-only, but even if you add them using read_only=False, the files are written only in the fake system (e.g. in memory). The real files are never changed.

add_real_file(), add_real_directory() and add_real_symlink() also allow you to map a file or a directory tree into another location in the fake filesystem via the argument target_path.

from pyfakefs.fake_filesystem_unittest import TestCase

class ExampleTestCase(TestCase):

    fixture_path = os.path.join(os.path.dirname(__file__), 'fixtures')
    def setUp(self):
        # make the file accessible in the fake file system

    def test_using_fixture1(self):
        with open(os.path.join(self.fixture_path, 'fixture1.txt') as f:
            # file contents are copied to the fake file system
            # only at this point
            contents =

Handling mount points

Under Linux and MacOS, the root path (/) is the only mount point created in the fake file system. If you need support for more mount points, you can add them using add_mount_point().

Under Windows, drives and UNC paths are internally handled as mount points. Adding a file or directory on another drive or UNC path automatically adds a mount point for that drive or UNC path root if needed. Explicitly adding mount points shall not be needed under Windows.

A mount point has a separate device ID (st_dev) under all systems, and some operations (like rename) are not possible for files located on different mount points. The fake file system size (if used) is also set per mount point.

Setting the file system size

If you need to know the file system size in your tests (for example for testing cleanup scripts), you can set the fake file system size using set_disk_usage(). By default, this sets the total size in bytes of the root partition; if you add a path as parameter, the size will be related to the mount point (see above) the path is related to.

By default, the size of the fake file system is considered infinite. As soon as you set a size, all files will occupy the space according to their size, and you may fail to create new files if the fake file system is full.

from pyfakefs.fake_filesystem_unittest import TestCase

class ExampleTestCase(TestCase):

    def setUp(self):

    def test_disk_full(self):
        with open('/foo/bar.txt', 'w') as f:
            self.assertRaises(OSError, f.write, 'a' * 200)

To get the file system size, you may use get_disk_usage(), which is modeled after shutil.disk_usage().

Pausing patching

Sometimes, you may want to access the real filesystem inside the test with no patching applied. This can be achieved by using the pause/resume functions, which exist in fake_filesystem_unittest.Patcher, fake_filesystem_unittest.TestCase and fake_filesystem.FakeFilesystem. There is also a context manager class fake_filesystem_unittest.Pause which encapsulates the calls to pause() and resume().

Here is an example that tests the usage with the pyfakefs pytest fixture:

from pyfakefs.fake_filesystem_unittest import Pause

def test_pause_resume_contextmanager(fs):
    fake_temp_file = tempfile.NamedTemporaryFile()
    assert os.path.exists(
    assert not os.path.exists(
    real_temp_file = tempfile.NamedTemporaryFile()
    assert os.path.exists(
    assert not os.path.exists(
    assert os.path.exists(

Here is the same code using a context manager:

from pyfakefs.fake_filesystem_unittest import Pause

def test_pause_resume_contextmanager(fs):
    fake_temp_file = tempfile.NamedTemporaryFile()
    assert os.path.exists(
    with Pause(fs):
        assert not os.path.exists(
        real_temp_file = tempfile.NamedTemporaryFile()
        assert os.path.exists(
    assert not os.path.exists(
    assert os.path.exists(


Modules not working with pyfakefs

Modules may not work with pyfakefs for several reasons. pyfakefs works by patching some file system related modules and functions, specifically:

  • most file system related functions in the os and os.path modules
  • the pathlib module
  • the build-in open function and
  • shutil.disk_usage

Other file system related modules work with pyfakefs, because they use exclusively these patched functions, specifically shutil (except for disk_usage), tempfile, glob and zipfile.

A module may not work with pyfakefs because of one of the following reasons:

  • It uses a file system related function of the mentioned modules that is not or not correctly patched. Mostly these are functions that are seldom used, but may be used in Python libraries (this has happened for example with a changed implementation of shutil in Python 3.7). Generally, these shall be handled in issues and we are happy to fix them.
  • It uses file system related functions in a way that will not be patched automatically. This is the case for functions that are executed while reading a module. This case and a possibility to make them work is documented above under modules_to_reload.
  • It uses OS specific file system functions not contained in the Python libraries. These will not work out of the box, and we generally will not support them in pyfakefs. If these functions are used in isolated functions or classes, they may be patched by using the modules_to_patch parameter (see the example for file locks in Django above), and if there are more examples for patches that may be useful, we may add them in the documentation.
  • It uses C libraries to access the file system. There is no way no make such a module work with pyfakefs - if you want to use it, you have to patch the whole module. In some cases, a library implemented in Python with a similar interface already exists. An example is lxml, which can be substituted with ElementTree in most cases for testing.

A list of Python modules that are known to not work correctly with pyfakefs will be collected here:

  • multiprocessing has several issues (related to points 1 and 3 above). Currently there are no plans to fix this, but this may change in case of sufficient demand.

If you are not sure if a module can be handled, or how to do it, you can always write a new issue, of course!

OS temporary directories

Tests relying on a completely empty file system on test start will fail. As pyfakefs does not fake the tempfile module (as described above), a temporary directory is required to ensure tempfile works correctly, e.g., that tempfile.gettempdir() will return a valid value. This means that any newly created fake file system will always have either a directory named /tmp when running on Linux or Unix systems, /var/folders/<hash>/T when running on MacOs and C:\Users\<user>\AppData\Local\Temp on Windows.

User rights

If you run pyfakefs tests as root (this happens by default if run in a docker container), pyfakefs also behaves as a root user, for example can write to write-protected files. This may not be the expected behavior, and can be changed. Pyfakefs has a rudimentary concept of user rights, which differentiates between root user (with the user id 0) and any other user. By default, pyfakefs assumes the user id of the current user, but you can change that using fake_filesystem.set_uid() in your setup. This allows to run tests as non-root user in a root user environment and vice verse. Another possibility is the convenience argument allow_root_user described above.