I'm trying to write an example where I'd like to use a frozen dataclass instance during type checking and swap it out with a normal dataclass to avoid paying the instantiation cost of frozen dataclasses.
The goal is to ensure that the instance is immutable with the type checker and use a regular dataclass during runtime. Here's the snippet:
from dataclasses import dataclass
from typing import TYPE_CHECKING
from functools import partial
if TYPE_CHECKING:
frozen = partial(dataclass, frozen=True)
else:
frozen = dataclass
@frozen
class Foo:
x: int
y: int
foo = Foo(1, 2) # mypy complains about the number of arguments
foo.x = 3 # instead, mypy should complain here
This works as expected during runtime, but running mypy raises this error. Pyright gives me the same error as well:
foo.py:49: error: Too many arguments for "Foo" [call-arg]
In this snippet, the type checker can catch the mutation error:
@dataclass(frozen=True)
class Foo:
x: int
y: int
foo = Foo(1, 2)
foo.x = 3 # mypy correctly catches the error here
So, I'm guessing that the type checker doesn't like when I'm aliasing frozen = dataclass or frozen = partial(...). How do I annotate this properly so that the type checker understands that it's a dataclass instance and doesn't complain about mismatched argument count?
P.S: This is just an exercise. I know turning on dataclass(frozen=True) is way easier, and I shouldn't care about performance in such cases. I was inspired to try this after reading a blog post by Tin Tvrtković on making attr class instances frozen at compile time.
Use @dataclass_transform with frozen_default = True:
if TYPE_CHECKING:
T = TypeVar('T')
@dataclass_transform(frozen_default = True)
def frozen(cls: type[T]) -> type[T]:
...
else:
frozen = dataclass
frozen_default was added in Python 3.12. However, since @dataclass_transform deliberately accepts all keyword arguments, 3.11 (exactly the same) and lower (using typing_extensions) will allow this just fine.
This works with both Mypy and Pyright:
reveal_type(Foo) # mypy => (x: int, y: int) -> Foo
# pyright => type[Foo]
foo = Foo(1, 2) # mypy + pyright => fine
foo.x = 3 # mypy => error: Property "x" defined in "Foo" is read-only
# pyright => error: Cannot assign member "x" for type "Foo"; "Foo" is frozen