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at runtime dataclass special-cases FunctionType, but at type-time mypy special-cases Callable, leading to mismatches #14869
Description
Bug Report
If you declare an attribute on a dataclass to be Callable, Mypy assumes that its __get__ will not be called, even on a default attribute. But this is only true if the default is a FunctionType, not some other type of callable.
Similarly, the inverse is true: an abstract type (such as a callable Protocol) will tell mypy that __get__ will be called, even though if the concrete default value is a FunctionType, it won't be.
Specifically, the following program contains 2 type errors, but MyPy does not think so:
To Reproduce
from __future__ import annotations
from typing import Callable, Protocol, ParamSpec, TypeVar, Generic
from dataclasses import dataclass
def f() -> None:
print("hello world")
def f2(self: object) -> None:
print("bound self", self)
P = ParamSpec("P")
T = TypeVar("T")
R = TypeVar("R", covariant=True)
T_con = TypeVar("T_con", contravariant=True)
class BindableMethod(Protocol[T_con, R]):
def __get__(self, instance: T_con, owner: None | type[object]) -> Callable[[], R]:
...
def __call__(me, self: T_con) -> R:
...
class UnboundNonFunction(Generic[P, R]):
def __call__(self) -> int:
print("unbound")
return 3
def __get__(self, instance: object, owner: type | None) -> BoundNonFunction:
print("binding", instance, owner)
return BoundNonFunction(instance)
@dataclass(frozen=True)
class BoundNonFunction:
instance: object
def __call__(self) -> str:
print("bound", self.instance)
return "wat"
@dataclass
class FuncHolder():
func: Callable[[], None] = f
func2: BindableMethod[FuncHolder, None] = f2
func3: Callable[[], int] = UnboundNonFunction()
FuncHolder().func()
try:
FuncHolder().func2()
except Exception as e:
print("whoops", str(e))
try:
print(FuncHolder().func3() + 4)
except Exception as e:
print("whoops 2", str(e))https://mypy-play.net/?mypy=latest&python=3.11&gist=1ac4e7c090774489207dfbc737de2d61
Expected Behavior
I would expect runtime and type-time behavior to be consistent here. Specifically I just don't expect __get__ to be called on things with a Callable type.
Actual Behavior
Mypy succeeds, rather than reporting the type errors.
Your Environment
- Mypy version used: mypy 1.1.1 (compiled: yes)
- Mypy command-line flags: none
- Mypy configuration options from
mypy.ini(and other config files): none - Python version used: 3.11.2 (python.org, macOS, aarch64)
Activity
zzzeek commentedon Mar 10, 2023
I think for pep681 it's really important to keep pyright in the loop because the reference implementation is that of pyright: https://peps.python.org/pep-0681/#reference-implementation
for the above test case pyright only reports:
/home/classic/dev/sqlalchemy/test3.py
/home/classic/dev/sqlalchemy/test3.py:21:18 - warning: Instance methods should take a "self" parameter (reportSelfClsParameterName)
for general pep-681 behavior you might want to raise it at https://github.com/python/peps/issues
ikonst commentedon Mar 12, 2023
It's not that dataclasses discern between "Callable" and "FunctionType", it's just that anything that quacks like a descriptor triggers the behavior described here.
Note the
__get__is only called a couple times, withinstance=None, meaning that it's not useful to e.g. bind a method to an instance. Maybe what you're hoping to do isn't even possible?It looks to me like accidental behavior that got documented, given how it's a mere side-effect of how the default is represented (as a class variable's value), and how changing from Python syntax
= dto= field(default=d)eliminates this behavior:class Descriptor: def __get__(self, instance, owner): return 42 d = Descriptor() @dataclass class C: - desc: Descriptor = d + desc: Descriptor = field(default=d) c = C() -assert c.desc == 42 +assert isinstance(c.desc, Descriptor)Right now the plugin does not checks for field's type vs. default's type — it kinda happens on it own (due to typeshed annotations), thought it might be doable.
But is it even worthwhile to replicate given how odd and accidental this feature seems?