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kw_only dataclass fields confuse mypy on argument order #19017

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inducer opened this issue May 2, 2025 · 0 comments · Fixed by #19018
Closed

kw_only dataclass fields confuse mypy on argument order #19017

inducer opened this issue May 2, 2025 · 0 comments · Fixed by #19018
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bug mypy got something wrong topic-dataclasses

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@inducer
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inducer commented May 2, 2025

Bug Report

This example (derived from #17731) runs flawlessly on Python 3.13:

from dataclasses import dataclass, field

@dataclass
class User:
    id: int = field(kw_only=True)
    name: str

u = User("Foo", id=0)
assert u.name == "Foo"

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&flags=strict&gist=ce6c431acd9f161f68d395b12cc966e9

Expected Behavior

Mypy should accept the code. Its output suggests it is confused about positional argument order (and their number).

Actual Behavior

Mypy 1.15 says:

main.py:8: error: "User" gets multiple values for keyword argument "id"  [misc]
main.py:8: error: Too few arguments for "User"  [call-arg]
main.py:8: error: Argument 1 to "User" has incompatible type "str"; expected "int"  [arg-type]
Found 3 errors in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: 1.15
  • Mypy command-line flags: none
  • Mypy configuration options from mypy.ini (and other config files): none
  • Python version used: 3.13
@inducer inducer added the bug mypy got something wrong label May 2, 2025
This was referenced May 2, 2025
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@sterliakov sterliakov mentioned this issue May 3, 2025
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sterliakov added a commit to sterliakov/mypy that referenced this issue Jun 4, 2025
@sterliakov
Merge remote-tracking branch 'upstream/master' into bugfix/pythongh-1…
61761e4 
…9017-dataclass-fields-kwonly-order
JukkaL pushed a commit that referenced this issue Jun 5, 2025
@sterliakov
Move dataclass kw_only fields to the end of the signature (#19018)
b147d11 
Fixes #19017. Fixes #17731.

This is a rather naive change: python does that at runtime. `kw_only`
args can be in any order, and non-kwonly args should remain sorted as-is
(stable sort). I don't understand why this was only done in presence of
a parent dataclass - AFAIC kwonly fields work that way since `kw_only`
was introduced in py3.10.

The test I changed was invalid and asserted a false positive to the best
of my knowledge.
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Move dataclass kw_only fields to the end of the signature sterliakov/mypy
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@inducer @sterliakov