How do I test the following code with unittest.mock:

def testme(filepath):
    with open(filepath) as f:
        return f.read()

The way to do this has changed in mock 0.7.0 which finally supports mocking the python protocol methods (magic methods), particularly using the MagicMock:

http://www.voidspace.org.uk/python/mock/magicmock.html

An example of mocking open as a context manager (from the examples page in the mock documentation):

>>> open_name = '%s.open' % __name__
>>> with patch(open_name, create=True) as mock_open:
...     mock_open.return_value = MagicMock(spec=file)
...
...     with open('/some/path', 'w') as f:
...         f.write('something')
...
<mock.Mock object at 0x...>
>>> file_handle = mock_open.return_value.__enter__.return_value
>>> file_handle.write.assert_called_with('something')