8000 Support types with custom `__instancecheck__` method · Issue #248 · dry-python/classes · GitHub
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sobolevn opened this issue Jul 5, 2021 · 0 comments
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Support types with custom __instancecheck__ method #248

sobolevn opened this issue Jul 5, 2021 · 0 comments
8000
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enhancement New feature or request

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@sobolevn
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sobolevn commented Jul 5, 2021

This code does not work:

from classes import typeclass

class Meta(type):
    def __instancecheck__(self, other) -> bool:
        return other == 1

class Some(object, metaclass=Meta):
    ...

@typeclass
def some(instance) -> bool:
    ...


@some.instance(Some)
def _some_some(instance: Some) -> bool:
    return True


print(some(1))
# NotImplementedError: Missing matched typeclass instance for type: int

But, it should! For example, phantom-types use this method: https://github.com/antonagestam/phantom-types/blob/main/phantom/base.py#L28-L43

If we want to support them - we would need this feature.

Solution

I suggest that we need to add types with __instancecheck__ defined to both ._instances and ._protocols.
This way both of these cases would work:

print(some(Some()))
print(some(1))
@sobolevn sobolevn added the enhancement New feature or request label Jul 5, 2021
sobolevn added a commit that referenced this issue Jul 5, 2021
@sobolevn
Closes #248
bcb1790
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