P

safir.asyncio.P = ~P

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]

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

P = ParamSpec('P')

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.