You can use a forward reference by using a string name for the not-yet-defined Client
class:
class Server():
def register_client(self, client: 'Client')
pass
As of Python 3.7, you can also postpone all runtime parsing of annotations by adding the following __future__
import at the top of your module:
from __future__ import annotations
at which point the annotations are stored as string representations of the abstract syntax tree for the expression; you can use typing.get_type_hints()
to resolve those (and resolve forward references as used above).
See PEP 563 — Postponed Evaluation of Annotations for details; this behaviour will be the default in Python 4.0.