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Combining Generic and NamedTuple #653

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DrPyser opened this issue Jun 22, 2019 · 6 comments
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Combining Generic and NamedTuple #653

DrPyser opened this issue Jun 22, 2019 · 6 comments
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topic: feature Discussions about new features for Python's type annotations

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@DrPyser
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DrPyser commented Jun 22, 2019

Currently, in python 3.7, it seems impossible to properly compose typing.NamedTuple and typing.Generic. That is, I want to do this:

import typing
T = typing.TypeVar("T")

class Test(typing.NamedTuple, typing.Generic[T]):
    a: T

So this works, the class can be used as a NamedTuple.
However:

x: Test[int] = Test(a=1) # TypeError: 'type' object is not subscriptable

Right, subclassing NamedTuple generates a class that inherits only from tuple.

Maybe this was discussed before, I've looked a bit in this repo and haven't found much, except #588, which doesn't really address this.

Is there any plan or possibility for this to be resolved? Maybe more generally, allowing NamedTuple to be composable with other types(I know this might be difficult because of metaclass conflicts).

Thanks!
@DrPyser
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DrPyser commented Jun 22, 2019 •
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Something of a workaround:

import typing
T = typing.TypeVar("T")
class Test(typing.NamedTuple):
    a: T
class Test(Test, typing.Generic[T]): pass

x: Test[int] = Test(a=1) # works fine

But unsurprisingly mypy won't accept this:

$ mypy /tmp/test.py 
/tmp/test.py:6: error: Invalid type "test.T"
/tmp/test.py:8: error: Name 'Test' already defined on line 5
/tmp/test.py:11: error: Generic tuple types not supported
/tmp/test.py:12: error: Generic tuple types not supported

@lemon24
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lemon24 commented Mar 4, 2020

It seems this is being worked on in python/mypy#685.

@illeatmyhat
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illeatmyhat commented Apr 29, 2020 •
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It's also impossible to do

from typing import TypedDict, TypeVar, Generic

T = TypeVar('T')
class Test(TypedDict, Generic[T]):
    a: T

with error
TypedDict cannot inherit from a non-TypedDict base class

@srittau
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srittau commented Nov 4, 2021

If a type checker doesn't support this, this seems to be more of a type checker issue than a general typing issue, so I'm closing this here.

@srittau srittau closed this as completed Nov 4, 2021
@Gobot1234
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Typing currently bans this in mro entries

@srittau srittau reopened this Nov 4, 2021
@srittau srittau added the topic: feature Discussions about new features for Python's type annotations label Nov 4, 2021
@AlexWaygood
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This is now:

So I think this feature request can be closed as "accepted"!

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@srittau @JelleZijlstra @lemon24 @illeatmyhat @DrPyser @Gobot1234 @AlexWaygood