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
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