Python 类型提示：return 基于属性的类型

Question

尝试类型提示，我在思考一些问题时遇到了一些麻烦：

class Node(object):
    credentials_class = credentials.BaseCredentials

    @property
    def credentials_instance(self) -> [credentials_class]:
        return self.credentials_class(self.credentials_data)


class OtherNode(Node):
    credentials_class = credentials.OtherCredentials  # subclass of BaseCredentials

我怎样才能使 OtherNode().credentials_instance returns credentials_class 中指定的类型，对于 Node 的所有子类？

credentials_class 属性不是必需的，如果我有其他方式告诉系统“此节点的 credentials_instance 属性 returns 此实例类型"，那就好。

Answer 1

答案是generics。通过使 Node 通用并使用凭据 class 进行参数化，您可以指定您希望它们工作的类型。缺点是，最顶层的 class 无法将默认值分配给 credentials_class，因为它会与通用冲突。

from typing import Type, Generic, TypeVar

CredentialsClassT = TypeVar("CredentialsClassT", bound=credentials.BaseCredentials)

class Node(Generic[CredentialsClassT]):
    credentials_class: Type[CredentialsClassT]

    @property
    def credentials_instance(self) -> CredentialsClassT:
        return self.credentials_class(self.credentials_data)

# example usage
class BaseNode(Node[credentials.BaseCredentials]):
    credentials_class = credentials.BaseCredentials

class OtherNode(Node[credentials.OtherCredentials]):
    credentials_class = credentials.OtherCredentials  # subclass of BaseCredentials

输入到位，例如

reveal_type(OtherNode.credentials_instance)

将使 MyPy 按预期打印 not: Revealed type is 'credentials.OtherCredentials'。

Python 类型提示：return 基于属性的类型

Python type hinting: return type based on attribute

python

python-typing