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Description
The overloads onlist.__add__ lead to divergent behavior betweenmypy andpyright when doing something as simple as a list concatenation
# Overloading looks unnecessary, but is needed to work around complex mypy problems@overloaddef__add__(self,value:list[_T],/)->list[_T]: ...@overloaddef__add__(self,value:list[_S],/)->list[_S|_T]: ...
Code sample inpyright playground,https://mypy-play.net/?mypy=latest&python=3.12&gist=abf6a8834020af17a16bd8cfb44b2f10
fromtypingimportAny,overloadclassListA[T]:# emulates builtins list@overloaddef__add__(self,other:"ListA[T]",/)->"ListA[T]":returnListA()@overloaddef__add__[S](self,other:"ListA[S]",/)->"ListA[T | S]":returnListA()classListB[T]:# without overloadsdef__add__[S](self,other:"ListB[S]",/)->"ListB[T | S]":returnListB()# mypy | pyrightreveal_type(list[str]()+list[str]() )# list[str] | list[str] ✅reveal_type(list[str]()+list[int]() )# list[str | int] | list[str | int] ✅reveal_type(list[str]()+list[Any]() )# list[Any] | list[str] ❌reveal_type(ListA[str]()+ListA[str]() )# ListA[str] | ListA[str] ✅reveal_type(ListA[str]()+ListA[int]() )# ListA[str | int] | ListA[str | int] ✅reveal_type(ListA[str]()+ListA[Any]() )# ListA[Any] | ListA[str] ❌reveal_type(ListB[str]()+ListB[str]() )# ListB[str] | ListB[str] ✅reveal_type(ListB[str]()+ListB[int]() )# ListB[str | int] | ListB[str | int] ✅reveal_type(ListB[str]()+ListB[Any]() )# ListB[str | Any] | ListB[str | Any] ✅
This ultimately causes some very annoying type errors when checking wrapper functions inpyright.
Code sample inpyright playground
fromtypingimportMapping,Any# function with 2 optional argumentsdeffoo(arg:object,/,*,opt1:str= ...,opt2:int= ...)->None: ...# wrapper that forwards args via dictdeffoo_wrapper(arg:object,foo_kwargs:Mapping[str,Any])->None:# apply new defaultsfoo_kwargs= {"opt1":"new_default"}|dict(foo_kwargs)foo(arg,**foo_kwargs)# "str" cannot be assigned to parameter "opt2"
PR#14282 and#14284 show mypy-primer results of simplifying the overloads away fromlist.__add__ anddict.__or__.
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