dataclass collection inference problem
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Description
Bug Report
It seems that when typing a collection of dataclasses, if they all happen to have the same fields, then the inferred type will be a Callable instead of type.
The example below is self-contained and demonstrates the problem very clearly.
To Reproduce
https://mypy-play.net/?mypy=latest&python=3.14&gist=5dfadfda23e846913cbee4d56568b69e
from dataclasses import Field, dataclass
from typing import ClassVar, Protocol, TypeVar
@dataclass
class C1:
foo: int = 0
@dataclass
class C2:
foo: int = 1
@dataclass
class C3:
foo: int = 2
bar: str = ""
class Dataclass(Protocol):
__dataclass_fields__: ClassVar[dict[str, Field]]
DataclassT = TypeVar("DataclassT", bound=Dataclass)
def f(a1: list[type[DataclassT]]): ...
reveal_type([C1, C2])
reveal_type([C1, C3])
f([C1, C2]) # this gives an error, but the others work
f([C1, C3])
f([C2, C3])
f([C1, C2, C3])
Expected Behavior
I would expect dataclasses to always be typed as classes instead of becoming callables because they happen share fields.
Actual Behavior
t.py:31: note: Revealed type is "builtins.list[def (foo: builtins.int =) -> builtins.object]"
t.py:32: note: Revealed type is "builtins.list[builtins.type]"
t.py:34: error: Value of type variable "DataclassT" of "f" cannot be "object" [type-var]
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.18.2 (but I tested 1.18.1, 1.17.1, 1.17.0 and they all have the same problem)
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): follow_untyped_imports = true - Python version used: 3.14.0 (also fails with 3.13.*)
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
Start with the self-contained reproducer in the issue and the linked mypy-play example, then trace mypy's type inference for list literals containing dataclass classes. Done means [C1, C2] is inferred as a list of class types rather than callables, and all shown calls to f type-check without the type-variable error.
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