How to Mock open() in Python Using unittest.mock.patch

import unittest
from unittest.mock import patch, mock_open


def apply_reverse_check(file_path, expected_patch):
    """
    Check if a patch can be reverse-applied by comparing file content
    with the expected patch's reverse result.
    """
    try:
        with open(file_path, "r") as f:
            content = f.read()
        if content == expected_patch["reverse"]:
            return "can reverse-apply"
        return "cannot reverse-apply"
    except FileNotFoundError:
        return "file missing"


class TestApplyReverseCheck(unittest.TestCase):
    @patch("builtins.open", new_callable=mock_open, read_data="old content")
    def test_reverse_check_success(self, mock_file):
        result = apply_reverse_check("file.txt", {"reverse": "old content"})
        self.assertEqual(result, "can reverse-apply")

    @patch("builtins.open", new_callable=mock_open, read_data="new content")
    def test_reverse_check_failure(self, mock_file):
        result = apply_reverse_check("file.txt", {"reverse": "old content"})
        self.assertEqual(result, "cannot reverse-apply")


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