python - Can you annotate return type when value is instance of cls? -

given class helper method initialization:

class trivialclass:     def __init__(self, str_arg: str):         self.string_attribute = str_arg      @classmethod     def from_int(cls, int_arg: int) -> ?:         str_arg = str(int_arg)         return cls(str_arg) 

is possible annotate return type of from_int method?

i'v tried both cls , trivialclass pycharm flags them unresolved references sounds reasonable @ point in time.

use generic type indicate you'll returning instance of cls:

from typing import type, typevar  t = typevar('t', bound='trivialclass')  class trivialclass:     # ...      @classmethod     def from_int(cls: type[t], int_arg: int) -> t:         # ...         return cls(...) 

any subclass overriding class method returning instance of parent class (trivialclass or subclass still ancestor) detected error, because factory method defined returning instance of type of cls.

the bound argument specifies t has (subclass of) trivialclass; because class doesn't yet exist when define generic, need use forward reference (a string name).

see annotating instance , class methods section of pep 484.

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note: first revision of answer advocated using forward reference naming class return value, issue 1212 made possible use generics instead, better solution.