ParamSpec

Parameter specification variable.

The preferred way to construct a parameter specification is via the
dedicated syntax for generic functions, classes, and type aliases,
where the use of '**' creates a parameter specification::

    type IntFunc[**P] = Callable[P, int]

The following syntax creates a parameter specification that defaults
to a callable accepting two positional-only arguments of types int
and str:

    type IntFuncDefault[**P = [int, str]] = Callable[P, int]

For compatibility with Python 3.11 and earlier, ParamSpec objects
can also be created as follows::

    P = ParamSpec('P')
    DefaultP = ParamSpec('DefaultP', default=[int, str])

Parameter specification variables exist primarily for the benefit of
static type checkers.  They are used to forward the parameter types of
one callable to another callable, a pattern commonly found in
higher-order functions and decorators.  They are only valid when used
in ``Concatenate``, or as the first argument to ``Callable``, or as
parameters for user-defined Generics. See class Generic for more
information on generic types.

An example for annotating a decorator::

    def add_logging[**P, T](f: Callable[P, T]) -> Callable[P, T]:
        '''A type-safe decorator to add logging to a function.'''
        def inner(*args: P.args, **kwargs: P.kwargs) -> T:
            logging.info(f'{f.__name__} was called')
            return f(*args, **kwargs)
        return inner

    @add_logging
    def add_two(x: float, y: float) -> float:
        '''Add two numbers together.'''
        return x + y

Parameter specification variables can be introspected. e.g.::

    >>> P = ParamSpec("P")
    >>> P.__name__
    'P'

Note that only parameter specification variables defined in the global
scope can be pickled.