python / python/mypy

Not detecting `enum.auto` vs `enum.auto()`

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bug topic-enum
Dominant language
Python
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Description

Bug Report

mypy is not detecting forgetting to call enum.auto() in IntEnum and StrEnum.

To Reproduce

from enum import IntEnum, StrEnum, auto

class SomeIntEnum(IntEnum):
    CORRECT_AUTO = auto()
    INCORRECT_AUTO = auto

reveal_type(SomeIntEnum.CORRECT_AUTO.value)
reveal_type(SomeIntEnum.INCORRECT_AUTO.value)

class SomeStrEnum(StrEnum):
    CORRECT_AUTO = auto()
    INCORRECT_AUTO = auto

reveal_type(SomeStrEnum.CORRECT_AUTO.value)
reveal_type(SomeStrEnum.INCORRECT_AUTO.value)

Running this code will actually crash with a TypeError, but mypy will pass:

  • For the IntEnum.INCORRECT_AUTO: TypeError: int() argument must be a string, a bytes-like object or a real number, not 'type'
  • For the StrEnum.INCORRECT_AUTO: TypeError: <class 'enum.auto'> is not a string

Expected Behavior

I expect mypy to error when one forgets to call auto, as:

  1. The code doesn't run
  2. Even if it did run, the enum's value wouldn't be a primitive as intended

Actual Behavior

mypy passes with the below output:

a.py:7:13: note: Revealed type is "builtins.int"
a.py:8:13: note: Revealed type is "def () -> enum.auto"
a.py:14:13: note: Revealed type is "builtins.str"
a.py:15:13: note: Revealed type is "def () -> enum.auto"

Your Environment

  • Mypy version used: 1.11.1
  • Mypy command-line flags: n/a
  • Mypy configuration options from mypy.ini (and other config files): n/a
  • Python version used: 3.12.4

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

Start by reproducing the report with the provided IntEnum and StrEnum examples on Python 3.12.4, then trace mypy's enum member and enum.auto handling to find where a function value is accepted without a diagnostic. Add coverage for both missing-call cases and verify that mypy reports errors while the called auto() cases retain their current revealed types.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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