---
id: pyfakefs
version: "6.2.0"
license: Apache-2.0
license_treatment: permissive
maintenance: active
---

# pyfakefs — Implements a fake file system that mocks the Python file system modules.

License: permissive · Maintenance: active · Downloads: 4.5M/mo

## What it is and what it does

pyfakefs is a testing library that replaces the real filesystem with an in-memory fake one during test execution. It automatically mocks Python's file system modules so your tests can create, read, and manipulate files without touching disk. Each test gets a fresh empty filesystem, making tests isolated and fast. The library works with unittest (via TestCase subclass or Patcher context manager) and as a decorator on individual tests.

The package handles cross-platform testing by emulating Linux, Windows, and macOS filesystems, and includes features like mapping real files into the fake filesystem, tracking filesystem size, pausing/resuming patching, and simulating non-root user behavior. It does not work with libraries that use C-level filesystem access, since pyfakefs cannot patch underlying C library calls.

Use it for:

- Test file I/O code (reading, writing, deleting) without creating temporary files on disk
- Verify error handling for missing files or permission issues in a controlled environment
- Run filesystem-heavy test suites in parallel without disk contention or cleanup overhead
- Test cross-platform file path logic (Windows vs. POSIX) on a single machine
- Simulate full filesystem scenarios (large directory trees, disk full conditions) cheaply in memory

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

pyfakefs mocks the Python file system modules to run tests against an in-memory fake filesystem instead of the real disk, with no changes required to the code under test.

Yes. pyfakefs is production-stable (Development Status 5), actively maintained, has no security vulnerabilities, and zero runtime dependencies. It solves a real testing problem—isolating filesystem operations—and is widely used. Install it if your test suite touches the filesystem.

## Install

pip install pyfakefs
uv add pyfakefs
poetry add pyfakefs

## Installing pyfakefs

Before you install: Low friction: pure Python wheel with no runtime dependencies. Actively maintained with recent releases; last commit 2026-08-13. Supports current Python versions on Linux, Windows, and macOS.

License in practice: Apache-2.0 (permissive): you can use, modify, and distribute pyfakefs freely in commercial and private projects, provided you include a copy of the license and state significant changes.

Quickstart:

pip install pyfakefs

from pyfakefs import fake_filesystem_unittest

class TestWithFakeFS(fake_filesystem_unittest.TestCase):
    def setUp(self):
        self.setUpPyfakefs()
    
    def test_file_operations(self):
        self.fs.create_file('/tmp/test.txt', contents='hello')
        with open('/tmp/test.txt') as f:
            self.assertEqual(f.read(), 'hello')

Requires Python 3.10 or later; C-based filesystem libraries cannot be mocked by pyfakefs.

Verify before relying:

- Performance characteristics when filesystem size grows or test count scales
- Compatibility with specific C-based libraries beyond the documented limitations
- Whether all test frameworks and plugins work seamlessly with pyfakefs patching

## Package facts

- License: Apache-2.0 (permissive)
- Python support: supports_current
- Install friction: low
- Maintenance: active
- Downloads: 4.5M/month (top 5,000 on PyPI)
- Known vulnerabilities: none known

## Tags

mock filesystem for testing, fake file system testing, in-memory filesystem testing, file system mocking library, test file operations without disk, unittest filesystem mocking, temporary filesystem per test, test-isolation, mocking, filesystem

[View on SkillFed](https://skillfed.io/packages/pyfakefs) · [View on PyPI](https://pypi.org/project/pyfakefs/)