Python unit tests with mock_open() and patch

I am having trouble patching the open method of my module with mock_open as suggested in the docs.

import my_module
import mock

def test_foo():
    m = mock.mock_open()
    with patch('my_module.open', m, create=True):
        my_module.io_func()

IOError: [Errno 2] No such file or directory: 'foofile'

Any idea why the patch isn't working?

Alternatively, I can get the following the execute as expected:

import my_module
import mock

def test_foo():
    m = mock.mock_open()
    my_module.__builtins__['open'] = m
    my_module.io_func()

However, this feels like a hack.