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 creates a new empty in-memory file system at each test start, which
replaces the real filesystem during the test. Think of pyfakefs as making a
per-test temporary directory, except for an entire file system.

There are several means to achieve this: by using the fs fixture if running
pytest, by using fake_filesystem_unittest.TestCase as a base class if using
unittest, by using a fake_filesystem_unittest.Patcher instance as a context
manager, or by using the patchfs decorator.