Usage ===== 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: .. code:: python from pyfakefs.fake_filesystem_unittest import TestCase class ExampleTestCase(TestCase): def setUp(self): self.setUpPyfakefs() def test_create_file(self): file_path = '/test/file.txt' self.assertFalse(os.path.exists(file_path)) self.fs.create_file(file_path) self.assertTrue(os.path.exists(file_path)) The usage is explained in more detail in :ref:`auto_patch` and demonstrated in the files ``example.py`` and ``example_test.py``. 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: .. code:: python def my_fakefs_test(fs): # "fs" is the reference to the fake file system fs.create_file('/var/data/xx1.txt') 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: .. code:: python 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 = f.read() You can also initialize ``Patcher`` manually: .. code:: python 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: 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()``. .. note:: 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 `pytest_fixture_test.py `__ with the example fixture in `conftest.py `__. We advice to use this example fixture code as a template for your customized pytest plugins. modules_to_reload ~~~~~~~~~~~~~~~~~ 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: .. code:: python import os import pathlib.Path - imported as another name: .. code:: python import os as my_os - imported using one of these two specially handled statements: .. code:: python from os import path from pathlib import Path Additionally, functions from file system related modules are patched automatically if imported like: .. code:: python from os.path import exists from os import stat This also works if importing the functions as another name: .. code:: python 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: .. code:: python 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: .. code:: python 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/sut.py``, the following code will work: .. code:: python # example using unittest class ReloadModuleTest(fake_filesystem_unittest.TestCase): def setUp(self): self.setUpPyfakefs(modules_to_reload=[example.sut]) def test_path_exists(self): file_path = '/foo/bar' self.fs.create_dir(file_path) self.assertTrue(example.sut.check_if_exists(file_path)) # example using Patcher def test_path_exists(): with Patcher() as patcher: file_path = '/foo/bar' patcher.fs.create_dir(file_path) assert example.sut.check_if_exists(file_path) Example using pytest: .. code:: python # conftest.py ... from example import sut @pytest.fixture def fs_reload_sut(): patcher = Patcher(modules_to_reload=[sut]) patcher.setUp() linecache.open = patcher.original_open tokenize._builtin_open = patcher.original_open yield patcher.fs patcher.tearDown() # test_code.py ... def test_path_exists(fs_reload_sut): file_path = '/foo/bar' fs_reload_sut.create_dir(file_path) assert example.sut.check_if_exists(file_path) modules_to_patch ~~~~~~~~~~~~~~~~ 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: .. code:: python 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__ parameter.""" # fs represents the fake filesystem; for a real example, it can be # saved here and used in the implementation pass @staticmethod def lock(f, flags): return True @staticmethod 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_django_stuff() # 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() ... additional_skip_names ~~~~~~~~~~~~~~~~~~~~~ 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``: .. code:: python with Patcher(additional_skip_names=['pydevd']) as patcher: patcher.fs.create_file('foo') Alternatively to the module names, the modules themselves may be used: .. code:: python import pydevd with Patcher(additional_skip_names=[pydevd]) as patcher: patcher.fs.create_file('foo') 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). allow_root_user ~~~~~~~~~~~~~~~ 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 I\O operations on the file (may be used to "fill up" the file system with large files). .. code:: python from pyfakefs.fake_filesystem_unittest import TestCase class ExampleTestCase(TestCase): def setUp(self): self.setUpPyfakefs() 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: self.assertEqual('test', f.read()) ``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``. .. code:: python from pyfakefs.fake_filesystem_unittest import TestCase class ExampleTestCase(TestCase): fixture_path = os.path.join(os.path.dirname(__file__), 'fixtures') def setUp(self): self.setUpPyfakefs() # make the file accessible in the fake file system self.fs.add_real_directory(self.fixture_path) 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 = f.read() 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. .. code:: python from pyfakefs.fake_filesystem_unittest import TestCase class ExampleTestCase(TestCase): def setUp(self): self.setUpPyfakefs() self.fs.set_disk_usage(100) 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: .. code:: python from pyfakefs.fake_filesystem_unittest import Pause def test_pause_resume_contextmanager(fs): fake_temp_file = tempfile.NamedTemporaryFile() assert os.path.exists(fake_temp_file.name) fs.pause() assert not os.path.exists(fake_temp_file.name) real_temp_file = tempfile.NamedTemporaryFile() assert os.path.exists(real_temp_file.name) fs.resume() assert not os.path.exists(real_temp_file.name) assert os.path.exists(fake_temp_file.name) Here is the same code using a context manager: .. code:: python from pyfakefs.fake_filesystem_unittest import Pause def test_pause_resume_contextmanager(fs): fake_temp_file = tempfile.NamedTemporaryFile() assert os.path.exists(fake_temp_file.name) with Pause(fs): assert not os.path.exists(fake_temp_file.name) real_temp_file = tempfile.NamedTemporaryFile() assert os.path.exists(real_temp_file.name) assert not os.path.exists(real_temp_file.name) assert os.path.exists(fake_temp_file.name) Troubleshooting --------------- 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 ``io.open`` - ``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//T`` when running on MacOs and ``C:\Users\\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.