How can I check which subclass an instance belongs to, without using isinstance?
(My understanding is that using isinstance is considered bad practice?...)
For example:

from typing import Protocol

class Entry(Protocol):
    name: str
    
    def is_file(self) -> bool: ...
    def is_folder(self) -> bool: ...

class File(Entry):
    name: str
    content: str

    def is_file(self) -> bool:
        return True
    def is_folder(self) -> bool:
        return False
    def get_size(self) -> int:
        return len(self.content)

class Folder(Entry):
    name: str
    children: list[Entry]
    
    def is_file(self) -> bool:
        return False
    def is_folder(self) -> bool:
        return True

def do_something(entry: Entry) -> None:
    if entry.is_file():
        print(entry.get_size())

On the last line of do_something the type checker understandably complains that entry has no method named get_size, because it obviously doesn't understand that I use is_file to to ensure that. (My type checker is Pylance in VSCode)
What can I do instead?
(I know this code works fine, but I would also like for it to pass type checking)

CodePudding user response：

There's no reason not to use isinstance here. It reduces the amount of unnecessary boilerplate. It isn't isinstance that is frowned up, but unnecessary run-time typechecking of any kind, including your custom is_* methods. Assuming there is a good reason for making do_something so unnecessarily generic, just use the following.

from typing import Protocol

class Entry:
    def __init__(self, *, name: str, **kwargs):
        super().__init__(**kwargs)
        self.name = name
    

class File(Entry):    
    def __init__(self, *, content: str, **kwargs):
        super().__init__(**kwargs)
        self.content = content

    def get_size(self) -> int:
        return len(self.content)


class Folder(Entry):    
    def __init__(self, *, children: list[Entry], **kwargs):
        super().__init__(**kwargs)
        self.children = children
    

def do_something(entry: Entry) -> None:
    if isinstance(entry, File):
        print(entry.get_size())

However, if do_something isn't supposed to work on anything except File objects, then say so in the type so you don't need any such type narrowing:

def do_something(entry: File) -> None:
    print(entry.get_size())

CodePudding user response：

A possibly more Pythonic way to handle this is to get rid of the is_file and is_folder completely. They're nothing but isinstance by another name.

class Entry:
    def get_size(self) -> int:
        raise NotImplementedError

class File(Entry):
    ...
    def get_size(self) -> int:
        return len(self.content)

class Folder(Entry):
    ...


def do_something(entry: Entry) -> None:
    try:
        print(entry.get_size())
    except NotImplementedError:
        pass