Dataclasses: `No overload variant of "asdict" matches argument type "Self" [call-overload]` when deciding decorator conditionally
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Description
Bug Report
Mypy raises a false positive for dataclass.asdict (No overload variant of "asdict" matches argument type "Self" [call-overload]) on the very specific circumstances:
- The specific decorator for
dataclassis decided based on a condition (e.g. Python version) - The
datalcasses.asdictmethod is called from a method that uses PEP 673's Self Type.
I also noted a similar problem occurs for dataclasses.replace: Value of type variable "_DataclassT" of "replace" cannot be "Self" [type-var].
To Reproduce
import sys
import dataclasses
import json
from typing import Self
if sys.version_info >= (3, 10):
dataclass = dataclasses.dataclass(kw_only=True)
else:
dataclass = dataclasses.dataclass
@dataclass
class HelloWorld:
a: int = 1
b: int = 2
c: int = 3
def double(self) -> Self:
fields = dataclasses.asdict(self)
return self.__class__(**{k: v*2 for k, v in fields.items()})
def json_overwrite(self, text: str) -> Self:
new = json.loads(text)
return dataclasses.replace(self, **new)
x = HelloWorld().double()
y = HelloWorld().json_overwrite('{"c": 42}')
assert x.b == 4
assert y.c == 42
print(f"{x=}")
print(f"{y=}")
Gist URL: https://gist.github.com/mypy-play/70b801fbe15391f0750be8f6c403fdf4
Playground URL: https://mypy-play.net/?mypy=latest&python=3.12&flags=show-error-context%2Cstrict&gist=70b801fbe15391f0750be8f6c403fdf4
Expected Behavior
There should be no false negative.
The program works fine in runtime:
$ python3.12 main.py
x=HelloWorld(a=2, b=4, c=6)
y=HelloWorld(a=1, b=2, c=42)
Actual Behavior
main.py: note: In member "double" of class "HelloWorld":
main.py:19: error: No overload variant of "asdict" matches argument type "Self" [call-overload]
main.py:19: note: Possible overload variants:
main.py:19: note: def asdict(obj: DataclassInstance) -> dict[str, Any]
main.py:19: note: def [_T] asdict(obj: DataclassInstance, *, dict_factory: Callable[[list[tuple[str, Any]]], _T]) -> _T
main.py: note: In member "json_overwrite" of class "HelloWorld":
main.py:24: error: Value of type variable "_DataclassT" of "replace" cannot be "Self" [type-var]
Found 2 errors in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.10.0
- Mypy command-line flags:
--show-error-context,--strict - Mypy configuration options from
mypy.ini(and other config files): - Python version used: 3.12
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Run the supplied Python 3.12 example with mypy 1.10.0 and --show-error-context --strict to reproduce the errors for dataclasses.asdict and dataclasses.replace. Start by tracing handling of the conditional dataclass decorator, Self, and the reported overload and type-variable checks. Done means the example type-checks without these false positives while preserving the runtime behavior.
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