Mock Python version with unittest.mock.patch

Use unittest.mock.patch to simulate a specific Python version and test version-dependent behavior.

Medium Python 3.8+ Aug 9, 2026 Modern tooling 13 views 0 copies

unittest mock version testing

Python code

16 lines

Python 3.8+

import sys
import unittest
from unittest.mock import patch

class TestPythonVersion(unittest.TestCase):
    @patch("sys.version_info", (3, 9, 0, "final", 0))
    def test_python_version_pinned(self):
        self.assertEqual(sys.version_info[:2], (3, 9))
        print(f"Pinned version: {sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}")

    def test_real_python_version(self):
        print(f"Actual version: {sys.version_info.major}.{sys.version_info.minor}.{sys.version_info.micro}")
        self.assertTrue(sys.version_info >= (3, 9))

if __name__ == "__main__":
    unittest.main()

Output

stdout

Pinned version: 3.9.0
.
Actual version: 3.10.12 (example)
.

----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK

How it works

The @patch("sys.version_info", (3, 9, 0, "final", 0)) decorator temporarily replaces sys.version_info with a fake tuple, letting you test code that branches on Python version. Since patch applies only to the decorated test method, other tests still see the real interpreter version. This is great for testing compatibility layers, conditional imports, or deprecated API warnings. To simulate different versions, just pass a different tuple—like (3, 11, 4, "final", 0).

Common mistakes

Patching the wrong path—must match where the object is used, not where it's defined.
Forgetting that `patch` as a decorator only affects the decorated test method.
Assuming `sys.version_info` is a simple tuple when it's actually a `sys.version_info` object; a tuple works fine for equality but may break attribute access beyond the first two elements.

Variations

Use `with patch("sys.version_info", (3, 8, 0, "final", 0)):` statement for conditional patching.
Use `unittest.mock.mock_version` to simulate version in third-party libraries.

Real-world use cases

Testing compatibility of your library against multiple Python versions in CI without separate jobs.
Verifying that code paths handling deprecated features behave correctly on older interpreters.
Ensuring feature flags or version-dependent imports activate the right module during testing.

Mock Python version with unittest.mock.patch

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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How it works

Common mistakes

Variations

Real-world use cases

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