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Re: Typing on child class' methods of a Generic base class

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From: pyt...@boiteameuh.org (Nicolas Haller)
Newsgroups: comp.lang.python
Subject: Re: Typing on child class' methods of a Generic base class
Date: Sat, 12 Mar 2022 12:05:43 -0500
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by: Nicolas Haller - Sat, 12 Mar 2022 17:05 UTC

On 2022-03-10 12:31, Dieter Maurer wrote:
> Nicolas Haller wrote at 2022-3-9 10:53 -0500:
>> ...
>> The documentation about "user-defined generic types"[1] says that I can
>> fix some types on a child class (class MyDict(Mapping[str, T]):) but
>> doesn't say much about the signature of the methods I need to
>> implement/override on that child class.
>
> I have the fealing that this is a case of (generic type) "specialization".
> In this setup, `Mapping` would be a generic type with two type
> variables `K` (the key type) and `V` (the value type).
> The signatures of its methods would use those type variables.
> In your example, you specialize the key type to `str` (and
> leave the value type generic). The signatures of the methods
> would automatically follow this specialization -- without the need
> to do anything.

If I understand correctly, you're saying I should use the first
alternative by keeping the signature as it is in the base class:
---
T = TypeVar("T")

class Base(Generic[T], metaclass=ABCMeta):
"""A base class."""

@abstractmethod
def _private_method(self, an_arg: T) -> T:
...

def public_method(self, an_arg: T) -> T:
from_private_method = self._private_method(an_arg)
return from_private_method

class Alternative1(Base[int]):
def _private_method(self, an_arg: T) -> T: # I keep T
return 42
---

The problem with it is that mypy doesn´t seem quite happy with it:
../scratch.py:22: error: Incompatible return value type (got "int",
expected "T")

Do you think this is error is incorrect? If so, I can open an issue with
mypy.

Thanks,

--
Nicolas

Life is a whim of several billion cells to be you for a while.

devel / comp.lang.python / Re: Typing on child class' methods of a Generic base class

Subject	Author
Re: Typing on child class' methods of a Generic base class	Nicolas Haller