mock open file

gistfile1.py

@contextmanager
def mock_open(filename, contents=None, complain=True):
    """Mock __builtin__.open() on a specific filename

    Let execution pass through to __builtin__.open() on other
    files. Return a BytesIO with :contents: if the file was matched. If
    the :contents: parameter is not given or if it None, a BytesIO
    instance simulating an empty file is returned.

    If :complain: is True (default). Will raise an error if a
    __builtin__.open was called with a file that was not mocked.

    """
    open_files = []  # simulate non-local scope with mutable lists
    def mock_file(*args):
        if args[0] == filename:
            r = BytesIO(contents)
            r.name = filename
        else:
            mocked_file.stop()
            r = open(*args)
            mocked_file.start()

        open_files.append(r)
        return r

    mocked_file = mock.patch('__builtin__.open', mock_file)
    mocked_file.start()
    try:
        yield
    except NotMocked as e:
        if e.filename != filename:
            raise
    mocked_file.stop()

    found = False
    for f in open_files:
        f.close()
        if f.name == filename:
            found = f
        elif complain:
            raise NotMocked(f.name)

    if not found:
        raise AssertionError("The file %s was not opened." % filename)

class NotMocked(Exception):
    def __init__(self, filename):
        super(NotMocked, self).__init__(
            "The file %s was opened, but not mocked." % filename)
        self.filename = filename