TypeIs for isinstance

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.

从链接的 scripts/check_docstrings.py 示例以及所请求的 isinstance/TypeIs 行为开始。明确单个检查和多个检查应如何缩小类型范围，然后确定验证该行为所需的 typeshed 存根和测试；当提案定义了明确的类型推断结果并包含相应的覆盖时，即视为完成。