How to mock a builtin method like 'open' in python?

Question

def file_handling():
    temp_file = open("/root/temp.tmp", 'w')
    temp_file.write("a")
    temp_file.write("b")

How to mock the 'open' method and subsequent write statements here? When i checked the solution online, suggestions were to use mock_open using the mock library. How can i make use of that here?

self.stubs.Set(__builtins__, "open", lambda *args: <some obj>) does not seem to work.

Welcome to [so]. What do you mean by "mock" and what methods would you allow yourself to use? — Qantas 94 Heavy, Dec 10 '13 at 12:22
I think "mock" is fairly well defined and unambiguous when it comes to programming - http://en.wikipedia.org/wiki/Mock_object — Tim, Dec 10 '13 at 12:25

score 1 · Answer 1 · answered Dec 10 '13 at 12:38

Well, using the mock library, I think this should work (not tested):

import mock
from unittest2 import TestCase

class MyTestCase(TestCase):
    def test_file_handling_writes_file(self):
        mocked_open_function = mock.mock_open():

        with mock.patch("__builtin__.open", mocked_open_function):
            file_handling()

        mocked_open_function.assert_called_once_with('/root/temp.tmp', 'w')
        handle = mocked_open_function()
        handle.write.assert_has_calls()

Thank you for reply. assert_has_calls() takes at least 2 arguments (1 given) what should be the mock.call value here? — chinmay, Dec 11 '13 at 11:58

How to mock a builtin method like 'open' in python?

1 Answers1