'Fake for open().
Verifies the arguments and returns a StringIO with the content listed
in self._files.'

@contextlib.contextmanager
def open_fake(self, filename, mode, encoding):

    assert (mode == 'r')
    assert (encoding == 'utf-8')
    if (self._files is None):
        raise OSError
    (yield io.StringIO(''.join(self._files[filename])))
