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Misleading warning for generic argument type on wrong return type #12156
Description
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: /
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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
Activity
[-]Misleading warning for argument type on wrong return type[/-][+]Misleading warning forgeneric argument type on wrong return type[/+][-]Misleading warning forgeneric argument type on wrong return type[/-][+]Misleading warning for generic argument type on wrong return type[/+]JelleZijlstra commentedon Feb 9, 2022
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 toint.if you first put the result of
coalescein a variable, I believe you'll get the result you expect.NiklasRosenstein commentedon Feb 23, 2022
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.
Extend special case for context-based typevar inference to typevar un…