python / python/mypy

Decorator with constrained TypeVar specializes to first constraint

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#20,437 3 comments 0 reactions 0 assignees View on GitHub

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Description

Bug Report

Changing the order of constraints of TypeVar T changes whether or not mypy passes. The wrapped method always gets narrowed to whatever the first constraint type is set to.

To Reproduce

from collections.abc import Callable
from typing import TypeVar, reveal_type


T = TypeVar("T", str, bytes)  # fails
# T = TypeVar("T", bytes, str)  # passes

def decorator(fn: Callable[[T], T]) -> Callable[[T], T]:
    def wrapped(data: T) -> T:
        return fn(data)
    return wrapped

@decorator
def process(data: T) -> T:
    return data

b: bytes = b"test"
result = process(b)

Or see mypy playground: https://mypy-play.net/?mypy=latest&python=3.12&gist=12fd2e05764d26a16cd1d16a10897671

Expected Behavior

Ideally pass for both versions, or consistently fail regardless of the constraint order.
On pyright playground the revealed types match my expectations and pass with either TypeVar order

Actual Behavior

mypy passes or fails depending on TypeVar constraint order, or the data input type

Your Environment

Same behavior with all tested mypy versions (1.6.0 .. 1.19.1) and python 3.11+

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 running the minimal decorator and constrained TypeVar reproducer from the issue, then trace mypy's handling of constraint specialization and the wrapped callable. Done means the result no longer depends on the order of the TypeVar constraints, with coverage for both constraint orders.

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
42/100

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