python / python/mypy

Error in mypy when using any() or all() to check for None in a iterable

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bug topic-type-narrowing
Dominant language
Python
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Description

Bug Report

Mypy raises errors indicating the absence of attributes for objects of type "None" when using any() or all().

To Reproduce

from dataclasses import dataclass


def get_hobby_description(hobby: str) -> ...: ...


def get_best_friend_info(best_friend: str) -> ...: ...


@dataclass
class Person:
    name: str
    age: int
    hobby: str | None = None
    best_friend: str | None = None


person = Person(name="name", age=0, hobby=None, best_friend=None)
if not all((person.hobby, person.best_friend)):
    raise ValueError

if not any((person.hobby, person.best_friend)):
    raise ValueError

hobby_description = get_hobby_description(person.hobby)
best_friend_info = get_best_friend_info(person.best_friend)

Expected Behavior

The code should be recognized as correct since it checks for the absence of None among the variables person.hobby and person.best_friend.

Actual Behavior

Mypy raises errors indicating the absence of attributes for objects of type "None" when using any() or all().

error: Argument 1 to "get_hobby_description" has incompatible type "str | None"; expected "str"  [arg-type]
error: Argument 1 to "get_best_friend_info" has incompatible type "str | None"; expected "str"  [arg-type]

Your Environment

  • Mypy version used: mypy 1.10.0 (compiled: yes)
  • Mypy command-line flags: No
  • Mypy configuration options from mypy.ini (and other config files): No
  • Python version used: 3.12.2

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

No source file or test is named. Start by running the provided dataclass reproducer with mypy 1.10.0 and inspect how any() and all() affect narrowing of optional attributes. Done means the reported behavior is resolved or its intended semantics are documented and covered by a regression test.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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