Wrangling the typing PEPs

Posted Dec 16, 2021 17:50 UTC (Thu) by anselm (subscriber, #2796)
In reply to: Wrangling the typing PEPs by mb
Parent article: Wrangling the typing PEPs

Does Python have the possibility to specify a Procotol as "type" instead of nailing down the type to some specific ones?

Yes, this is explained in PEP-544, which deals with “duck typing”.

There's List, which means object with list procotol, as far as I understand it. But is there something similar for paths?

The os module implements PathLike as an “abstract base class” for classes that implement path-like behaviour (such as pathlib.Path). Basically this means that such classes support the __fspath__() method, which is supposed to return a representation of the object that is a str suitable to use as a file system path. If you wanted to say that a function accepts a parameter that is either a str to begin with, or a PathLike object, you could define it as

from os import fspath, PathLike

def do_something_with_path(path: str | PathLike) -> …:
    …
    f = open(fspath(path))
    …

(where os.fspath(s) will return s outright if s is of type str, or else s.__fspath__()). Of course you could also define a type variable like

PathOrStr = TypeVar('PathOrStr', str, PathLike)

and then simply use this as

def do_something_with_path(path: PathOrStr) -> ...:
    …

Can the developer create new such types for his own protocols?

Absolutely. See here.