Type hints: solve circular dependency [duplicate]

Question

This question already has answers here:

How do I specify that the return type of a method is the same as the class itself? (4 answers)

Closed 5 months ago.

The following produces NameError: name 'Client' is not defined. How can I solve it?

class Server():
    def register_client(self, client: Client)
        pass


class Client():
    def __init__(self, server: Server):
        server.register_client(self)

Answer 1

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.

Answer 2

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 typing module to satisfy these types of forward/circular dependency issues.

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

from typing import TypeVar, Generic

T_Server = TypeVar('T_Server')
T_Client = TypeVar('T_Client')


class Server(Generic[T_Server]):
    clients: list = None

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

    def register_client(self, client: T_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: T_Server) -> T_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: T_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()