Get a unique id given a query_string

def get_unique_document_id(query_str):
    # type: (str) -> str
    """Get a unique id given a query_string"""
    assert isinstance(query_str, string_types), (
        "Must receive a string as query_str. Received {}"
    ).format(repr(query_str))

    if query_str not in _cached_queries:
        _cached_queries[query_str] = sha1(str(query_str).encode("utf-8")).hexdigest()
    return _cached_queries[query_str]