Example:
        >>> # DISABLE_DOCTEST
        >>> dict1 = {1:1, 2:2, 3:3}
        >>> dict2 = {2:3, 3:3}
        >>> assert_keys_are_subset(dict1, dict2)
        >>> #dict2 = {4:3, 3:3}

def assert_keys_are_subset(dict1, dict2):
    """
    Example:
        >>> # DISABLE_DOCTEST
        >>> dict1 = {1:1, 2:2, 3:3}
        >>> dict2 = {2:3, 3:3}
        >>> assert_keys_are_subset(dict1, dict2)
        >>> #dict2 = {4:3, 3:3}
    """
    keys1 = set(dict1.keys())
    keys2 = set(dict2.keys())
    unknown_keys = keys2.difference(keys1)
    assert len(unknown_keys) == 0, 'unknown_keys=%r' % (unknown_keys,)