def inspect_format_method(callable):
    if not has_format:
        return None
    if not isinstance(callable, (types.MethodType,
                                 types.BuiltinMethodType)) or \
       callable.__name__ != 'format':
        return None
    obj = callable.__self__
    if isinstance(obj, string_types):
        return obj
The software uses a function that accepts a format string as an argument, but the format string originates from an external source.