Usage¶
Test Scenarios¶
There are several approaches for 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):
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 Automatically find and patch file functions and modules 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. The plugin
is registered for pytest on installing pyfakefs
as usual for pytest
plugins, so you can just use it:
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')
If you are bothered by the pylint
warning,
C0103: Argument name "fs" doesn't conform to snake_case naming style
(invalid-name)
,
you can define a longer name in your conftest.py
and use that in your
tests:
@pytest.fixture
def fake_filesystem(fs): # pylint:disable=invalid-name
"""Variable name 'fs' causes a pylint warning. Provide a longer name
acceptable to pylint for use in tests.
"""
yield fs
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 = f.read()
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 fake_filesystem_unittest.patchfs decorator¶
This is basically a convenience wrapper for the previous method.
If you are not using pytest
and want to use the fake filesystem for a
single function, you can write:
from pyfakefs.fake_filesystem_unittest import patchfs
@patchfs
def test_something(fake_fs):
# access the fake_filesystem object via fake_fs
fake_fs.create_file('/foo/bar', contents='test')
Note that fake_fs
is a positional argument and the argument name does
not matter. If there are additional mock.patch
decorators that also
create positional arguments, the argument order is the same as the decorator
order, as shown here:
@patchfs
@mock.patch('foo.bar')
def test_something(fake_fs, mocked_bar):
...
@mock.patch('foo.bar')
@patchfs
def test_something(mocked_bar, fake_fs):
...
Note
Avoid writing the patchfs
decorator between mock.patch
operators,
as the order will not be what you expect. Due to implementation details,
all arguments created by mock.patch
decorators are always expected to
be contiguous, regardless of other decorators positioned between them.
Caution
In previous versions, the keyword argument fs has been used instead, which had to be positioned after all positional arguments regardless of the decorator order. If you upgrade from a version before pyfakefs 4.2, you may have to adapt the argument order.
You can also use this to make a single unit test use the fake fs:
class TestSomething(unittest.TestCase):
@patchfs
def test_something(self, fs):
fs.create_file('/foo/bar', contents='test')
Customizing patching¶
fake_filesystem_unittest.Patcher
provides a few arguments to adapt
patching for cases where it does not work out of the box. These arguments
can also be used with unittest
and pytest
.
Using custom arguments¶
The following sections describe how to apply these arguments in different scenarios, using the argument allow_root_user as an example.
Patcher¶
If you use the Patcher
directly, you can just pass the arguments in the
constructor:
from pyfakefs.fake_filesystem_unittest import Patcher
with Patcher(allow_root_user=False) as patcher:
...
Unittest¶
If you are using fake_filesystem_unittest.TestCase
, the arguments can be
passed to setUpPyfakefs()
, which will pass them to the Patcher
instance:
from pyfakefs.fake_filesystem_unittest import TestCase
class SomeTest(TestCase):
def setUp(self):
self.setUpPyfakefs(allow_root_user=False)
def testSomething(self):
...
Pytest¶
In case of pytest
, you have two possibilities:
The standard way to customize the
fs
fixture is to write your own fixture which uses thePatcher
with arguments as has been shown above:
import pytest
from pyfakefs.fake_filesystem_unittest import Patcher
@pytest.fixture
def fs_no_root():
with Patcher(allow_root_user=False) as patcher:
yield patcher.fs
def test_something(fs_no_root):
...
You can also pass the arguments using
@pytest.mark.parametrize
. Note that you have to provide all Patcher arguments before the needed ones, as keyword arguments cannot be used, and you have to addindirect=True
. This makes it less readable, but gives you a quick possibility to adapt a single test:
import pytest
@pytest.mark.parametrize('fs', [[None, None, None, False]], indirect=True)
def test_something(fs):
...
patchfs¶
If you use the patchfs
decorator, you can pass the arguments directly to
the decorator:
from pyfakefs.fake_filesystem_unittest import patchfs
@patchfs(allow_root_user=False)
def test_something(fake_fs):
...
List of custom arguments¶
Following is a description of the optional arguments that can be used to
customize pyfakefs
.
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:
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 at least two specific cases where this is the case:
Initializing a default argument with a file system function is not patched automatically due to performance reasons (though it can be switched on using patch_default_args):
import os
def check_if_exists(filepath, file_exists=os.path.exists):
return file_exists(filepath)
If initializing a global variable using a file system function, the initialization will be done using the real file system:
from pathlib import Path
path = Path("/example_home")
In this case, path
will hold the real file system path inside the test.
The same is true, if a file system function is used in a decorator (this is
an example from a related issue):
import pathlib
@click.command()
@click.argument('foo', type=click.Path(path_type=pathlib.Path))
def hello(foo):
pass
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 function check_if_exists
shown above is located in
the file example/sut.py
, the following code will work:
import example
# 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 pytest
@pytest.mark.parametrize('fs', [[None, [example.sut]]], indirect=True)
def test_path_exists(fs):
file_path = '/foo/bar'
fs.create_dir(file_path)
assert example.sut.check_if_exists(file_path)
# example using Patcher
def test_path_exists():
with Patcher(modules_to_reload=[example.sut]) as patcher:
file_path = '/foo/bar'
patcher.fs.create_dir(file_path)
assert example.sut.check_if_exists(file_path)
# example using patchfs decorator
@patchfs(modules_to_reload=[example.sut])
def test_path_exists(fs):
file_path = '/foo/bar'
fs.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 (note that this
has now been been integrated into pyfakefs
):
class FakeLocks:
"""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(self)
...
# test code using pytest
@pytest.mark.parametrize('fs', [[None, None,
{'django.core.files.locks': FakeLocks}]], indirect=True)
def test_django_stuff(fs):
...
# test code using patchfs decorator
@patchfs(modules_to_patch={'django.core.files.locks': FakeLocks})
def test_django_stuff(fake_fs):
...
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 needs to access
the file system after test setup. To make this possible, the affected module
can be added to additional_skip_names
:
with Patcher(additional_skip_names=['pydevd']) as patcher:
patcher.fs.create_file('foo')
Alternatively to the module names, the modules themselves may be used:
import pydevd
with Patcher(additional_skip_names=[pydevd]) as patcher:
patcher.fs.create_file('foo')
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
.
use_known_patches¶
Some libraries are known to require patching in order to work with
pyfakefs
.
If use_known_patches
is set to True
(the default), pyfakefs
patches
these libraries so that they will work with the fake filesystem. Currently, this
includes patches for pandas
read methods like read_csv
and
read_excel
, and for Django
file locks–more may follow. Ordinarily,
the default value of use_known_patches
should be used, but it is present
to allow users to disable this patching in case it causes any problems. It
may be removed or replaced by more fine-grained arguments in future releases.
patch_open_code¶
Since Python 3.8, the io
module has the function open_code
, which
opens a file read-only and is used to open Python code files. By default, this
function is not patched, because the files it opens usually belong to the
executed library code and are not present in the fake file system.
Under some circumstances, this may not be the case, and the opened file
lives in the fake filesystem. For these cases, you can set patch_open_code
to PatchMode.ON
. If you just want to patch open_case
for files that
live in the fake filesystem, and use the real function for the rest, you can
set patch_open_code
to PatchMode.AUTO
:
from pyfakefs.fake_filesystem_unittest import PatchMode
@patchfs(patch_open_code=PatchMode.AUTO)
def test_something(fs):
...
Note
This argument is subject to change or removal in future
versions of pyfakefs
, depending on the upcoming use cases.
patch_default_args¶
As already mentioned, a default argument that is initialized with a file system function is not patched automatically:
import os
def check_if_exists(filepath, file_exists=os.path.exists):
return file_exists(filepath)
As this is rarely needed, and the check to patch this automatically is quite
expansive, it is not done by default. Using patch_default_args
will
search for this kind of default arguments and patch them automatically.
You could also use the modules_to_reload option with the module that
contains the default argument instead, if you want to avoid the overhead.
use_cache¶
If True (the default), patched and non-patched modules are cached between tests to avoid the performance hit of the file system function lookup (the patching itself is reverted after each test). As this is a new feature, this argument allows to turn it off in case it causes any problems:
@patchfs(use_cache=False)
def test_something(fake_fs):
fake_fs.create_file("foo", contents="test")
...
Please write an issue if you encounter any problem that can be fixed by using this parameter. Note that this argument may be removed in a later version, if no problems come up.
If you want to clear the cache just for a specific test instead, you can call
clear_cache
on the Patcher
or the fake_filesystem
instance:
def test_something(fs): # using pytest fixture
fs.clear_cache()
...
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, TestCase.fs
for unittest
, and the fs
argument
for the patchfs
decorator.
File creation helpers¶
To create files, directories or symlinks together with all the directories
in the path, you may use create_file()
, create_dir()
,
create_symlink()
and create_link()
, 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, see also Setting the file system size).
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()
.
create_symlink
and create_link
behave like os.symlink
and
os.link
, with any missing parent directories of the link created
automatically.
Caution
The first two arguments in create_symlink
are reverted in relation to
os.symlink
for historical reasons.
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):
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()
You can do the same using pytest
by using a fixture for test setup:
import pytest
import os
fixture_path = os.path.join(os.path.dirname(__file__), 'fixtures')
@pytest.fixture
def my_fs(fs):
fs.add_real_directory(fixture_path)
yield fs
def test_using_fixture1(my_fs):
with open(os.path.join(fixture_path, 'fixture1.txt') as f:
contents = f.read()
When using pytest
another option is to load the contents of the real file
in a fixture and pass this fixture to the test function before passing
the fs
fixture.
import pytest
import os
@pytest.fixture
def content():
fixture_path = os.path.join(os.path.dirname(__file__), 'fixtures')
with open(os.path.join(fixture_path, 'fixture1.txt') as f:
contents = f.read()
return contents
def test_using_file_contents(content, fs):
fs.create_file("fake/path.txt")
assert content != ""
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 set to 1 TB (which for most tests can be considered as 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):
self.setUpPyfakefs()
self.fs.set_disk_usage(100)
def test_disk_full(self):
with open('/foo/bar.txt', 'w') as f:
with self.assertRaises(OSError):
f.write('a' * 200)
f.flush()
To get the file system size, you may use get_disk_usage()
, which is
modeled after shutil.disk_usage()
.
Suspending 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(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:
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)
Simulating other file systems¶
Pyfakefs
supports Linux, MacOS and Windows operating systems. By default,
the file system of the OS where the tests run is assumed, but it is possible
to simulate other file systems to some extent. To set a specific file
system, you can change pyfakefs.FakeFilesystem.os
to one of
OSType.LINUX
, OSType.MACOS
and OSType.WINDOWS
. On doing so, the
behavior of pyfakefs
is adapted to the respective file system. Note that
setting this causes the fake file system to be reset, so you should call it
before adding any files.
Setting the os
attributes changes a number of pyfakefs.FakeFilesystem
attributes, which can also be set separately if needed:
is_windows_fs
- ifTrue
a Windows file system (NTFS) is assumed
is_macos
- ifTrue
andis_windows_fs
isFalse
, the standard MacOS file system (HFS+) is assumedif
is_windows_fs
andis_macos
areFalse
, a Linux file system (something like ext3) is assumed
is_case_sensitive
is set toTrue
under Linux and toFalse
under Windows and MacOS by default - you can change it to change the respective behavior
path_separator
is set to\
under Windows and to/
under Posix,alternative_path_separator
is set to/
under Windows and toNone
under Posix–these can also be adapted if needed
The following test works both under Windows and Linux:
from pyfakefs.fake_filesystem import OSType
def test_windows_paths(fs):
fs.os = OSType.WINDOWS
assert r"C:\foo\bar" == os.path.join('C:\\', 'foo', 'bar'))
assert os.path.splitdrive(r"C:\foo\bar") == ("C:", r"\foo\bar")
assert os.path.ismount("C:")
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
andos.path
modulesthe
pathlib
modulethe build-in
open
function andio.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 themodules_to_patch
parameter (see the example for file locks in Django above), or by usingunittest.patch
if you don’t need to simulate the functions. We added some of these patches topyfakefs
, so that they are applied automatically (currently done for somepandas
andDjango
functionality).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 islxml
, which can be substituted withElementTree
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.
subprocess has very similar problems and cannot be used with
pyfakefs
to start a process.subprocess
can either be mocked, if the process is not needed for the test, or patching can be paused to start a process if needed, and resumed afterwards (see this issue).Modules that rely on
subprocess
ormultiprocessing
to work correctly, e.g. need to start other executables. Examples that have shown this problem include GitPython and plumbum.the Pillow image library does not work with pyfakefs at least if writing JPEG files (see this issue)
pandas (the Python data analysis library) uses its own internal file system access written in C. Thus much of
pandas
will not work withpyfakefs
. Having said that,pyfakefs
patchespandas
so that many of theread_xxx
functions, includingread_csv
andread_excel
, as well as some writer functions, do work with the fake file system. If you use only these functions,pyfakefs
will work withpandas
.
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!
Pyfakefs behaves differently than the real filesystem¶
There are basically two kinds of deviations from the actual behavior:
unwanted deviations that we didn’t notice–if you find any of these, please write an issue and will try to fix it
behavior that depends on different OS versions and editions–as mentioned in Limitations,
pyfakefs
uses the TravisCI systems as reference system and will not replicate all system-specific behavior
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, or
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 to run tests as non-root user in a root user environment
is the convenience argument allow_root_user.
Pyfakefs and mock_open¶
If you patch open
using mock_open
before the initialization of
pyfakefs
, it will not work properly, because the pyfakefs
initialization relies on open
working correctly.
Generally, you should not need mock_open
if using pyfakefs
, because you
always can create the files with the needed content using create_file
.
This is true for patching any filesystem functions - avoid patching them
while working with pyfakefs
.
If you still want to use mock_open
, make sure it is only used while
patching is in progress. For example, if you are using pytest
with the
mocker
fixture used to patch open
, make sure that the fs
fixture is
passed before the mocker
fixture to ensure this.