Mocking open raises AttributeError

I am trying to translate mock_open unittest’s example but I get an AttributeError:

def test_open(mocker):
    with mocker.patch('__main__.open', mocker.mock_open(read_data='bibble')) as m:
        with open('foo') as h:
            result = h.read()

    m.assert_called_once_with('foo')
    assert result == 'bibble'

Output:

    def test_open(mocker):
        open_mock = mocker.mock_open(read_data='bibble')
>       with mocker.patch('builtins.open', open_mock) as m:
E       AttributeError: __enter__

NOTE: pytest-mock==1.10.0

[nicoddemus]

The purpose of the mocker fixture is to not use it as a context manager, otherwise you might as well use mock.patch directly. 😉

Try this:

def test_open(mocker):
    m = mocker.patch('__main__.open', mocker.mock_open(read_data='bibble'))
    with open('foo') as h:
        result = h.read()

    m.assert_called_once_with('foo')
    assert result == 'bibble'

See Note about usage as context manager in the README for more information.

I’m closing this but feel free to follow up with more questions. 👍

[MarcAragones]

I had to change '__main__.open' with 'builtins.open'.