Typing-sig September 2021

typing-sig@python.org

20 participants
13 discussions

Definition-site variance for TypeVar
by Sergei Lebedev 07 Sep '21

07 Sep '21

Hi everyone, Type variables allow to specify their variance at construction time, i.e variance is currently a property of a type variable. This is problematic for cases when a type variable is used outside of a generic class. For example, T_co = TypeVar("T", covariant=True) def callback(value: T_co): ... or ListT = list[T_co] I am curious, if we could instead deprecate the variance parameters and make variance local to the generic class definition. Specifically, * all type variables are created invariant; * a type variable can be annotated as covariant and contravariant using unary + and - operators respectively; * these operators are only allowed when a type variable is referenced in the bases of a class definition. For example, here T = TypeVar("T") class Source(Generic[+T]): ... the class Source is covariant in T. Wdyt? Sergei

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Re: Definition-site variance for TypeVar
by Gobot1234 07 Sep '21

07 Sep '21

There is a pep currently being written to support this, go to https://github.com/python/typing/issues/813 for the discussion about this.

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How to extend Type or how to type a wrapper class
by Patrick Arminio 05 Sep '21

05 Sep '21

Hi everyone! I'm trying to type a decorator that returns a Wrapper class of another class. The idea is that I have a wrapper class that only adds a couple of properties to the original class, something like this: ```python from typing import Type, TypeVar, Generic _T = TypeVar("_T") class Wrapper(Generic[_T]): wrap: Type[_T] enhanced: bool = True def __init__(self, cls: Type[_T]) -> None: self.wrap = cls def __call__(self, *args, **kwargs): ... def enhance(cls: Type[_T]) -> Type[_T]: return Wrapper(cls) # type: ignore class X: ... EnhanceX = enhance(X) reveal_type(EnhanceX) reveal_type(EnhanceX.enhanced) ``` The user using the class doesn't really need to know that it has been "enhanced", as it should work as it was before. My initial idea was to extend `Type` from typings, but that's not possible. What would you suggest to do here? Should I type all the methods on `Wrapper` to basically behave as if they were from `self.wrap`? not sure if that's actually possibile There's also a possibility where I'm doing something silly with what I'm trying to do, but it should make sense for my use case 😊

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Typing-sig September 2021