def METHOD_NAME(self, arg, previous_named, variables=None):
    name, value = split_from_equals(arg)
    if value is None:
        return False
    if variables:
        try:
            name = variables.replace_scalar(name)
        except DataError:
            return False
    spec = self._argspec
    return bool(previous_named or
                spec.var_named or
                name in spec.positional_or_named or
                name in spec.named_only)