Function _check_generic_specialization

Lib/typing.py:310–338

Check correct count for parameters of a generic cls (internal helper). This gives a nice error message in case of count mismatch.

(cls, arguments)

def _check_generic_specialization(cls, arguments):
    """Check correct count for parameters of a generic cls (internal helper).

    This gives a nice error message in case of count mismatch.
    """
    expected_len = len(cls.__parameters__)
    if not expected_len:
        raise TypeError(f"{cls} is not a generic class")
    actual_len = len(arguments)
    if actual_len != expected_len:
        # deal with defaults
        if actual_len < expected_len:
            # If the parameter at index `actual_len` in the parameters list
            # has a default, then all parameters after it must also have
            # one, because we validated as much in _collect_type_parameters().
            # That means that no error needs to be raised here, despite
            # the number of arguments being passed not matching the number
            # of parameters: all parameters that aren't explicitly
            # specialized in this call are parameters with default values.
            if cls.__parameters__[actual_len].has_default():
                return

            expected_len -= sum(p.has_default() for p in cls.__parameters__)
            expect_val = f"at least {expected_len}"
        else:
            expect_val = expected_len

        raise TypeError(f"Too {'many' if actual_len > expected_len else 'few'} arguments"
                        f" for {cls}; actual {actual_len}, expected {expect_val}")


def _unpack_args(*args):