How to Mock pathlib Path.read_text with mock_open in Python

Mock pathlib.Path.read_text using patch and mock_open to test file-reading code without touching the filesystem.

Easy Python 3.9+ Aug 9, 2026 Testing & modern typing 14 views 0 copies

testing mock pathlib unit-testing

Python code

14 lines

Python 3.9+

import pathlib
from unittest.mock import mock_open, patch

def read_config(filepath: pathlib.Path) -> str:
    """Read file content with pathlib."""
    return filepath.read_text()

if __name__ == "__main__":
    mock_data = "version: 1.0\nname: demo-app"
    
    with patch("pathlib.Path.open", mock_open(read_data=mock_data)):
        path = pathlib.Path("/fake/config.yaml")
        content = read_config(path)
        print(content)

Output

stdout

version: 1.0
name: demo-app

How it works

The patch context manager replaces Path.open with a mock that returns a file object pre-loaded with mock_data. Since read_text() internally calls open(), the patch intercepts that call and returns the fake content without any real I/O. The test can then assert against the returned string, verifying the logic that consumes the file content.

Common mistakes

Patching `pathlib.Path.read_text` directly instead of `pathlib.Path.open` – the latter is what `read_text` uses.
Forgetting to pass `mock_open(read_data=...)` – without it, the mock file returns empty bytes.
Using the wrong patch target when the file is opened elsewhere in the codebase.

Variations

Patch `builtins.open` when the code uses the standard `open()` instead of `pathlib.Path.read_text`.
Use `unittest.mock.patch` with a custom `MagicMock` that has a `read_text` method returning a fixed string.

Real-world use cases

Unit-testing a configuration loader that uses pathlib to read YAML or JSON settings files.
Testing a script that reads template files at startup to avoid filesystem dependencies in CI.
Validating error handling when a file is missing, without creating or deleting actual test fixtures.

How to Mock pathlib Path.read_text with mock_open in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Testing & modern typing

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