Example of mocking os.path.isfile

test_mock.py

#!/usr/bin/env python3
#
# Use py.test:
#  $ pytest test_mock.py

from unittest.mock import patch
import os.path


def my_isfile(filename):
    """Test function which calls os.path.isfile
    """
    if os.path.isfile(filename):
        return "Yes"
    else:
        return "Wrong"


@patch('os.path.isfile')
def test_isfile_with_return_value(mock_isfile):
    """Mocking os.path.isfile and using return_value
    """
    mock_isfile.return_value = True
    assert my_isfile('bla') == 'Yes'


@patch('os.path.isfile')
def test_isfile_with_side_effects(mock_isfile):
    """Mocking os.path.isfile with using side_effect
    """
    def side_effect(filename):
        if filename == 'foo':
            return True
        else:
            return False

    mock_isfile.side_effect = side_effect
    assert my_isfile('foo') == 'Yes'
    assert my_isfile('bla') == 'Wrong'