Shouldn't a slice of a bytestring be a bytestring?

[dvarrazzo]

Slicing bytes, bytearray, memoryview objects, the result is an object of the same type.

However, slicing ByteArray, mypy complains:

from collections.abc import ByteString

def f(s: ByteString) -> ByteString:
    return s[10:20]

mypy test-bytestring-slice.py
test-bytestring-slice.py:4: error: Incompatible return value type (got "Sequence[int]", expected "ByteString")

Tested with mypy 0.982, 0.990 (from git).

Is this a typeshed shortcoming?

[JelleZijlstra]

Don't use ByteString, it's basically useless. It declares no methods.

The behavior you see is because ByteString inherits __getitem__ from Sequence, and Sequence.__getitem__ doesn't guarantee that you get the same kind of Sequence back.

[dvarrazzo]

Ok.

Mine was an attempt to replace the homegrown Buffer type in psycopg (defined as Union[bytes, bytearray, memoryview] - something supporting the Buffer protocol) with something from the stdlib.

I guess we'll keep our Buffer 😄

[hauntsaninja]

Yeah, would love to see the back of ByteString python/cpython#91896

[JelleZijlstra]

In typeshed we have our own Buffer alias with a bunch more types: https://github.com/python/typeshed/blob/main/stdlib/_typeshed/__init__.pyi#L236. You can use it at type checking type (if TYPE_CHECKING: from _typeshed import ReadableBuffer).