pytest: mock builtin file open()

conftest.py

# Prerequisites:
#   $ pip install pytest pytest-mock

from functools import cache
from unittest.mock import MagicMock, mock_open

import pytest
from pytest_mock import MockerFixture


class FileMock(MagicMock):

    def __init__(self, mocker: MagicMock = None, **kwargs):
        super().__init__(**kwargs)

        if mocker:
            self.__dict__ = mocker.__dict__
            # configure mock object to replace the use of open(...)
            # note: this is useful in scenarios where data is written out
            _ = mock_open(mock=self)

    @property
    def read_data(self):
        return self.side_effect

    @read_data.setter
    def read_data(self, mock_data: str):
        """set mock data to be returned when `open(...).read()` is called."""
        self.side_effect = mock_open(read_data=mock_data)

    @property
    @cache
    def write_calls(self):
        """a list of calls made to `open().write(...)`"""
        handle = self.return_value
        write: MagicMock = handle.write
        return write.call_args_list

    @property
    def write_lines(self) -> str:
        """a list of written lines (as a string)"""
        return ''.join([c[0][0] for c in self.write_calls])


@pytest.fixture
def mock_file_open(mocker: MockerFixture) -> FileMock:
    return FileMock(mocker.patch('builtins.open'))

test_it_works.py

from unittest.mock import call


def test_mock_file_open_and_read(mock_file_open):
    mock_file_open.read_data = 'hello\nworld!'

    with open('/my/file/here', 'r') as in_file:
        assert in_file.readlines() == ['hello\n', 'world!']

    mock_file_open.assert_called_with('/my/file/here', 'r')


def test_mock_file_open_and_write(mock_file_open):
    with open('/out/file/here', 'w') as f:
        f.write('hello\n')
        f.write('world!\n')
        f.write('--> testing 123 :-)')

    mock_file_open.assert_called_with('/out/file/here', 'w')

    assert call('world!\n') in mock_file_open.write_calls

    assert mock_file_open.write_lines == """\
hello
world!
--> testing 123 :-)
""".rstrip()