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From: ram@zedat.fu-berlin.de (Stefan Ram)
Newsgroups: comp.lang.python
Subject: Re: Struggling to understand Callable type hinting
Date: 18 Jan 2025 16:11:05 GMT
Organization: Stefan Ram
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Expires: 1 Jan 2026 11:59:58 GMT
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References: <mailman.87.1737156810.2912.python-list@python.org> <Callable-20250118125426@ram.dialup.fu-berlin.de>
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ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:
>    @staticmethod
>    def _check_eof(method: Callable[Concatenate['BufferScanner', P], T]) -> Callable[Concatenate['BufferScanner', P], bool]:
>        def wrapper(self: 'BufferScanner', *args: P.args, **kwargs: P.kwargs) -> bool:

  "P" (ParamSpec) is resolved when the decorator is applied to any
  method. 

  - "P" (ParamSpec) captures all parameters of the decorated
    method except for the first "self" parameter.
    "Concatenate['BufferScanner', P]" then adds the
    "BufferScanner" type as the first parameter, effectively
    representing the complete parameter list of the method.

  "T" (TypeVar) is inferred based on the return type of the method
  being decorated.

  So, those type variables are resolved based on the specific
  method the decorator is applied to, allowing for generic and
  reusable type annotations across different classes and methods

  (This "resolution" of a type name corresponds to the
  initialization of a traditional name. But a single type name
  can have multiple values when the wrapper is being applied
  to multiple methods - one value per application.)