Type hints: solve circular dependency

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.

If you are on Python 3.7+, use from __future__ import annotations as mentioned in another answer. However, if you cannot use 3.7 yet due to OS limitation (like Cygwin as of 2019-06-03), you can use Forward References module to satisfy these types of forward/circular dependency issues.

Pardon the contrived example but this should illustrate the usefulness of this methodology.

class Server():
clients: list = None


def __init__(self):
self.clients=[]


def register_client(self, client: 'Client') -> None:
self.clients.append(client)
print('Client `%s` registered with server' % client.name)


def print_clients(self) -> None:
for i, client in enumerate(self.clients):
print('client %i: %s' % (i, client.name))


@staticmethod
def build_clone(server: 'Server') -> 'Server':
svr_new: Server = Server()
for client in server.clients:
svr_new.register_client(client)
return svr_new


class Client():
name: str = None
def __init__(self, name: str, server: 'Server'):
self.name = name
server.register_client(self)




svr = Server()
cli = Client('foo', svr)
svr.print_clients()


svr_clone = Server.build_clone(svr)
svr_clone.print_clients()