我怎么能在皮顿的普通类里找到一个可选的TypeVar?-allqahome-开发者的问答家园

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我怎么能在皮顿的普通类里找到一个可选的TypeVar?

原标题：How can I have an optional TypeVar in a Generic class in Python?

I m trying to write a simple type wrapper to represent the interface of decorator functions: from typing import Protocol, TypeVar, Generic TIn = TypeVar( TIn , contravariant=True) TOut = TypeVar( TOut , covariant=True) class Decorator(Protocol, Generic[TIn, TOut]): """ Represents a decorated value, used to simplify type definitions """ def __call__(self, value: TIn) -> TOut: ... This would be used to type a decorator function as follows: IntFunction = Callable[[int, int], int] def register_operator(op: str) -> Decorator[IntFunction, IntFunction]: def inner(value: IntFunction) -> IntFunction: # register the function or whatever return value return inner @register_operator("+") def add(a: int, b: int) -> int: return a + b In the above example, Mypy is able to validate the type signature of add to ensure it matches the specification of register_operator. This is useful for decorators that transform the type (eg converting it from an IntFunction to a StrFunction), but in almost all cases, TIn is identical to TOut, and so I want to simplify the usage of my definition. Essentially, I want to make it so that if TOut isn t given, it will be assumed to be the same as TIn, which would allow the above decorator function to be simplified to def register_operator(op: str) -> Decorator[IntFunction]: # Simplification here ^ def inner(value: IntFunction) -> IntFunction: # register the function or whatever return value return inner The ideal syntax I would use in my protocol definition would be something like this: class Decorator(Protocol, Generic[TIn, TOut = TIn]): """ Represents a decorated value, used to simplify type definitions """ def __call__(self, value: TIn) -> TOut: ... Note that this does not work. How can I achieve this functionality, whilst continuing to have the assurance that Mypy provides? I am happy to make the definition of Decorator as complex as needed, but I want to keep its simple usage.

最佳回答

So as far I know python doesn t support default values in generics, a generic that isn t fully defined becomes unknown. Perhaps it should support it, there s a draft pep for this feature (see update below). For now, I would create two classes that expose different interfaces. This is almost as good, it just means you need a second class: from typing import Protocol, TypeVar, Generic, overload TIn = TypeVar( TIn , contravariant=True) TOut = TypeVar( TOut , covariant=True) TSym = TypeVar( TSym ) class Decorator(Protocol, Generic[TIn, TOut]): """ Represents a decorated value, used to simplify type definitions """ def __call__(self, value: TIn) -> TOut: ... class SymmetricDecorator(Decorator[TSym,TSym], Generic[TSym], Protocol): pass By "Symmetric" I just mean the callable receives and returns the same type - there might be a better name. Hope this is useful! UPDATE: I ve since found there exists an active pep for default values for TypeVar: https://peps.python.org/pep-0696/

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