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Misleading warning for generic argument type on wrong return type #12156

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NiklasRosenstein opened this issue Feb 9, 2022 · 2 comments
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bug mypy got something wrong topic-type-context Type context / bidirectional inference

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@NiklasRosenstein
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NiklasRosenstein commented Feb 9, 2022

Bug Report

It appears Mypy propagates types into the type arguments of a function call if the types on the left side are know, slightly obscuring the actual error.

To Reproduce

Run mypy a.py with

import typing as t

T = t.TypeVar('T')
U = t.TypeVar('U')

def coalesce(a: T | None, b: U) -> t.Union[T, U]:
  if a is None:
    return b
  return a

def foo() -> int:
  return coalesce(42, None)

Expected Behavior

Looking at this, I would expect Mypy to complain that the declared return value of foo() (being int) does not match what is actually returned (int | None).

Actual Behavior

It appears that Mypy propagates the return type into the coalesce() type arguments instead and expects them both to be int, thus complaining about the argument type instead.

a.py: note: In function "foo":
a.py:15: error: Argument 2 to "coalesce" has incompatible type "None"; expected "int"  [arg-type]
      return coalesce(42, None)
                          ^
Found 1 error in 1 file (checked 1 source file)

I find this rather unintuitive. My suggestion would be to make Mypy complain about the type of the returned value instead of the argument type.

Your Environment

  • Mypy version used: 0.931

  • Mypy command-line flags: /

  • Mypy configuration options from pyproject.toml (and other config files):

    [tool.mypy]
    pretty = true
    warn_redundant_casts = true
    #warn_unused_ignores = true
    warn_no_return = true
    warn_unreachable = true
    show_error_context = true
    show_error_codes = true
  • Python version used: 3.10.2

  • Operating system and version: WSL 2, Ubuntu 20

@NiklasRosenstein NiklasRosenstein added the bug mypy got something wrong label Feb 9, 2022
@NiklasRosenstein NiklasRosenstein changed the title Misleading warning for argument type on wrong return type Misleading warning forgeneric argument type on wrong return type Feb 9, 2022
@NiklasRosenstein NiklasRosenstein changed the title Misleading warning forgeneric argument type on wrong return type Misleading warning for generic argument type on wrong return type Feb 9, 2022
@JelleZijlstra
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This is presumably a consequence of mypy's "type context" heuristic: it expects you to return an int, so it typechecks the coalesce() call in a way that sets the return type to int.

if you first put the result of coalesce in a variable, I believe you'll get the result you expect.

@NiklasRosenstein
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Hey @JelleZijlstra , you are right, that is a functioning workaround. There are others, but the issue is not that it can't be worked around but that when encountered it is confusing at first sight to understand what is happening. Working around it after the culprit is identified is not big of a deal.

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Labels
bug mypy got something wrong topic-type-context Type context / bidirectional inference
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