类型提示：解决循环依赖

Question

以下生成 NameError: name 'Client' is not defined。我该如何解决？

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


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

Answer 1

您可以通过使用 字符串 名称来使用 forward reference 尚未定义的 Client class:

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

As of Python 3.7，您还可以通过在模块顶部添加以下 __future__ 导入来推迟注释的 all 运行时解析：

from __future__ import annotations

此时注释被存储为表达式的抽象语法树的字符串表示；您可以使用 typing.get_type_hints() 来解决这些问题（并解决上面使用的前向引用）。

详情见PEP 563 -- Postponed Evaluation of Annotations；此行为将成为 Python 4.0.

中的默认行为

Answer 2

如果您使用的是 Python 3.7+，请使用 from __future__ import annotations 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 模块来解决这些类型的 forward/circular 依赖问题。

请原谅这个人为的例子，但这应该可以说明这种方法的有用性。

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()

类型提示：解决循环依赖

Type hints: solve circular dependency

python

type-hinting

python-typing