unittest with mock __builtin__.open

test_mock_open.py

import mock
import unittest

class TestOpen(unittest.TestCase):
    def test_mock_open(self):
        m = mock.patch('__builtin__.open', 
                mock.mock_open(read_data="some text"))
        m.start()

        with open('somefile.txt', 'rb') as fin:
                text = fin.read()

        assert text == "some text"

        m.stop()