Helps mock the open context manager (or any contextmanager!) and does some doublechecking that the mock was used properly

mock_helpers.py

from contextlib import contextmanager
from nose.tools import assert_equals
from mock import patch, Mock
from StringIO import StringIO

@contextmanager
def mock_io_open(content, *args):
    stream = StringIO(content)
    with mock_context_manager('io.open', stream, *args) as mock:
        yield mock

@contextmanager
def mock_context_manager(mock_target, yielded_thing, *args):
    if not yielded_thing:
        yielded_thing = Mock()

    with patch(mock_target) as context_mock:
        context_mock.return_value.__enter__.return_value = yielded_thing  # mock the contextmanager
        yield yielded_thing
    assert context_mock.called, "%s mock wasn't used!" % mock_target
    if args:
        assert_equals(args, context_mock.call_args[0])  # make sure the filenames match

if __name__ == "__main__":
    import io
    with mock_io_open("foo\nbar\n"):
        with io.open('example') as thefile:
            assert_equals(thefile.readlines(), ["foo\n", "bar\n"])

    with mock_io_open('', "/root/myfile", 'w') as output:
        with io.open('/root/myfile', 'w') as thefile:
            thefile.write("foo\n")
            thefile.writelines(["bar\n", "baz\n"])
    assert_equals("foo\nbar\nbaz\n", output.getvalue())