Python类型提示-如何提示默认不为None的可选参数？

Question

I am trying to implement PEP-484 in my Python 3 code for practice. While working on the following practice question, which looks like:

def fetch_n(what: str, n="all") -> List[obj]:
    query = "some sql string"
    if n == "all":
        # do the fetching
    elif isinstance(n, int):
        query = query + " LIMIT ?"
        # do the fetching
    else:
        raise ValueError

Is it possible to hint n in the function definition to be -- const str or int ? If yes, how to do it?

I read the cheat-sheet and currently I am using from typing import Optional and n: Optional[int] but its not working as desired.

Answer 1

The Optional[X] is still only a type hint - it means X or None . Perhaps here you'd need an Union instead:

def fetch_n(what: str, n: Union[str, int] = "all") -> List[obj]:
    ...