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bytes broader than bytearray? #8970

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gsakkis opened this issue Oct 23, 2022 · 3 comments
Closed

bytes broader than bytearray? #8970

gsakkis opened this issue Oct 23, 2022 · 3 comments

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@gsakkis
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gsakkis commented Oct 23, 2022

Mypy errors with Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader [misc] for the following class:

from typing import Optional, Union, overload


class Transform:
    @overload
    def apply(self, b: bytes) -> bytes:
        """Return the transformed bytestring of the input"""

    @overload
    def apply(self, b: bytearray) -> None:
        """Transform the input bytearray in place"""

    def apply(self, b: Union[bytes, bytearray]) -> Optional[bytes]:
        pass

Changing the order of the two @overload doesn't give the error.

mypy --version
mypy 0.981 (compiled: yes)
@srittau
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srittau commented Oct 23, 2022

Yes, bytes includes bytearray in argument types: https://docs.python.org/3/library/typing.html?highlight=typing#typing.ByteString.

@srittau srittau closed this as not planned Won't fix, can't repro, duplicate, stale Oct 23, 2022
@gsakkis
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gsakkis commented Oct 23, 2022

So how is someone supposed to differentiate between the (actual) bytes vs bytearray types?

def f(b: Union[bytes, bytearray]) -> None:
    if isinstance(b, bytes):
        print("bytes")
    else:
        print("bytearray")

f(bytearray())

Mypy claims Statement is unreachable for the else clause but it is clearly reachable.

@AlexWaygood
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You may be interested in PEP 688 (written by @JelleZijlstra), which attempts to address this problem. The PEP is discussed here: https://discuss.python.org/t/pep-688-take-2-making-the-buffer-protocol-accessible-in-python/19756/1.

This problem was also recently discussed here: python/mypy#12643.

Regardless, this isn't really a typeshed issue; there's nothing we can do about it in this repo :)

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@srittau @gsakkis @AlexWaygood