When to patch

The patch method of the unittest.mock library has excellent documentation how it works.

Recently a co-worker asked me, when to use it.

Patching is something to use with unit tests, or more specific:

• When you only want to test function/class/method A, and not the other functions/classes used inside A, you should patch

test_unit.py

class TestCsvWriteFactory:
    @patch("app.download.service.generator.csv")
    def test_csv_writer_factory(self, mock_csv: MagicMock) -> None:
        """Patch the csv module, so it is not called and excluded from the test, 
        but assert it's write method is called
        """
        csv_writer_factory()
        mock_csv.writer.assert_called_once()

unit.py

import csv
from typing import Any

def csv_writer_factory():
    class FileLike:
        """An object that implements just the write method of the file-like
        interface.
        """

        def write(self, value: Any) -> Any:
            """Write the value by returning it"""
            return value

    return csv.writer(FileLike(), delimiter=",", quotechar='"')