Support narrowing using in keyword

[kumaraditya303]

See this code:

# pyright: strict


def func(browser: object):
    if isinstance(browser, str):
        reveal_type(browser)
    if browser in ("chrome", "firefox"):
        reveal_type(browser)

Pyright is able to narrow in first case of isinstance but not in second case. This is very popular pattern in python so worth to be considered.

Objects which should be supported in initial implementation:

• set
• tuple
• list
• deque

[erictraut]

FWIW, mypy doesn't currently provide narrowing support for this pattern. There is an open issue, but it hasn't received many upvotes yet. Here is another related feature request.

We'll wait to see if others upvote this request. In the meantime, you can implement your own user-defined type guard function using the PEP 647 functionality.