TypeIs for isinstance

[OutSquareCapital]

Apologies if I'm missing something obvious, but I checked issues and PR's and haven't seen this mentioned explicitly.

Could isinstance use TypeIs?

With multiples checks it could return an union, with one type T a TypeIs[T].

I often find myself writing one-liner helpers functions that just call isinstance, see this script for example:

https://github.com/OutSquareCapital/pyochain/blob/master/scripts/check_docstrings.py

This both add boilerplate and a performance tax due to a double function call overhead.

I'm very often using lambdas with fluent interfaces, e.g

res = (
my_iterator
.map(lambda x: ...)
.filter(lambda x: isinstance(x, int)
)

Assuming my_iterator is an Iterator[Any], it would be fantastic if it could be narrowed to Iterator[int] without having to define a separate TypeIs function.

[Andrew5057]

TypeIs wouldn't fix this problem; it occurs at the lambda definition, which doesn't retain the type-guarding properties of the wrapped function. Consider this demo (mypy):

from collections.abc import Iterator
from typing import TypeIs

def is_int(obj: object) -> TypeIs[int]:
    return isinstance(obj, int)

def int_filter(it: Iterator[object]) -> Iterator[int]:
    return filter(is_int, it)

def lambda_int_filter(it: Iterator[object]) -> Iterator[int]:
    return filter(lambda x: is_int(x), it)

The bare is_int gets inferred correctly, but the lambda-wrapped one doesn't.