Recursive sequence type of Sequence[NotAString] should not accept str

With at least mypy 1.13.0, the false negative can be reproduced with:

from collections.abc import Sequence
from typing import TypeAlias

class Scalar: ...

Vector: TypeAlias = Sequence[Scalar]
Tensor: TypeAlias = Vector | Sequence["Tensor"]

accepted_vector: Vector = [Scalar()]     # true negative
rejected_vector: Vector = "duck"         # true positive: ducks aren't vectors

accepted_tensor: Tensor = [[[Scalar()]]] # true negative
rejected_tensor1: Tensor = object()      # true positive: objects aren't tensors
rejected_tensor2: Tensor = "duck"        # false negative: ducks are tensors...?

mypy-play
pyright-play (works as expected)

originally posted in #731 (comment)

This works as expected (even if surprising) because str is a subclass of Sequence[str]. As an exercise, try writing a code where an actual runtime error happens because of this behavior (the point is that you can't).

implement a workaround for python/mypy#18184

This works as expected (even if surprising) because str is a subclass of Sequence[str].

but str is not assignable to Scalar

edit: your argument assumes an infinite horizon, but within the context of typing, I think that a finite horizon would be more appropriate, analogous to how there's also a limit on the recursion depth at runtime

edit edit: if you'd parametrize str on its length then that will allow you to define an inductive basecase as str[1], and Sequence[str[1]] otherwise; so there's also no need for an infinite horizon

As an exercise, try writing a code where an actual runtime error happens because of this behavior (the point is that you can't).

>>> import numpy as np
>>> np.array("not a tensor", np.float64)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: 'not a tensor'

FYI, this works as expected in pyright:

https://pyright-play.net/?strict=true&code=GYJw9gtgBAxmA28CmMAuBLMA7AzgOgEMAjGKdCABzBFSgGUkBHAVySxiQChRIpUBPCuiwBzMpWq0AKoKQBBeOgI5OnGPGU56MAhpAAuKHmOqAailTVDMivMXKoAXnpNW7JAG06OvQF1OUmw4VlA2dkpazuZo1FAAPi4sbBweAESBuNSp-pwEMBwUqEgAJgD6AG4WIdGWIE5QXj4EIAAUAJS%2BUF1dAMR8IKxQWEgiBBiVnCBIAFYWJRVVBlA1sc6pxcwwANap3Xu9-YNUOOjjSIYb21rNbADktJUxICq5%2BUiF80WZSxnBdc4eQHeXTNdq%2BcFQPqoAZIIYjMboCZTWZoT5BagARms6P%2BUDARBRqHa%2B0hh1hx1OiPOeIJFmuUyw9z4OJUyLmZS%2BfwATNjvvV1psdiSDsBdDhYcNRmcLoL6bDOdR8MYAPycIA

@ilevkivskyi

@ilevkivskyi I also feel like this is a Mypy issue. I am passing AFAIK all of Mypy's relevant recursion tests (except the ones with --disable-recursive-aliases) and this case seems to behave correct in my implementation.

@KotlinIsland

@KotlinIsland any thoughts on this?

I think its consistent with the concept/spec, but impractical... maybe. I think pyright is wrong/inconsistent here...

let's change how it works in basedmypy 🚀🚀🚀

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Recursive sequence type of `Sequence[NotAString]` should not accept `str` #18184

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Recursive sequence type of Sequence[NotAString] should not accept str #18184

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ilevkivskyi commented on Nov 24, 2024

jorenham commented on Nov 24, 2024

jorenham commented on Nov 24, 2024

jorenham commented on Nov 25, 2024

davidhalter commented on Nov 25, 2024

jorenham commented on Nov 25, 2024

KotlinIsland commented on Nov 26, 2024

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Recursive sequence type of `Sequence[NotAString]` should not accept `str` #18184