Tuple types don't work with higher level typing #16935

fjarri · 2024-02-21T21:44:47Z

mypy 1.8.0, python 3.10.10

So tuple types get matched and checked normally in simple situations like this:

a: tuple[int, int] = (2, "a")
# Incompatible types in assignment ...

But imagine I want to define a function that takes a type and returns a value of that type (e.g. by deserializing something as the given type):

from typing import Tuple, TypeVar, Type, List

_T = TypeVar("_T")

def structure(structure_into: Type[_T], obj: str) -> _T:
    raise NotImplementedError

This works with builtin types, or with generics like List:

reveal_type(structure(int, "a"))
# Revealed type is "builtins.int"
reveal_type(structure(List[int], "a"))
# Revealed type is "builtins.list[builtins.int]"

... but not with tuples:

reveal_type(structure(Tuple[int, int], "a"))
# note: Revealed type is "Never"
# error: Argument 1 to "structure" has incompatible type "object"; expected "type[Never]"  [arg-type]

Trying to reveal the type of Tuple itself leads to strange results too (compared to List[...]):

reveal_type(Tuple[int, int])
# Revealed type is "builtins.object"
reveal_type(tuple[int, int])
# Revealed type is "def [_T_co] (typing.Iterable[_T_co`1] =) -> builtins.tuple[_T_co`1, ...]"

So the question is, why does Tuple get a special treatment, and is there a way to make it behave like other generics?

The text was updated successfully, but these errors were encountered:

fjarri · 2024-03-15T22:58:13Z

Same issue with UnionType types (T1 | T2 | ...).

urnest · 2024-04-25T06:00:48Z

am having a go at this in https://github.com/urnest/mypy/tree/issue-16935

... so e.g. reveal_type(tuple[int, int]) gives expected def (p0: builtins.int, p1: builtins.int) -> tuple[builtins.int, builtins.int] ... rather than def [_T_co] (typing.Iterable[_T_co`1] =) -> builtins.tuple[_T_co`1, ...]

…back i.e. the mypy type of the expression union[str, int] is a CallableType with fallback builtins.type this matches the mypy type of the expression int which is a CallableType with fallback builtins.type (suggested by PR review discussion)

…back ... the original fallback ABCMeta is "correct" in that it aligns with the (abstract) base class of builtins.tuple. But rather than explicitly specify abc.ABCMeta as fallback, use the same fallback as the type we're applying arguments to (and that is abc.ABCMeta in our "tuple" case)

fjarri added the bug mypy got something wrong label Feb 21, 2024

urnest linked a pull request May 12, 2024 that will close this issue

fix #16935 fix type of tuple[X,Y] expression #17235

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Tuple types don't work with higher level typing #16935

Tuple types don't work with higher level typing #16935

fjarri commented Feb 21, 2024

fjarri commented Mar 15, 2024

urnest commented Apr 25, 2024

Tuple types don't work with higher level typing #16935

Tuple types don't work with higher level typing #16935

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fjarri commented Feb 21, 2024

fjarri commented Mar 15, 2024

urnest commented Apr 25, 2024