如何在元类中键入提示动态设置的类属性？

Question

当我动态设置一个类的属性时：

from typing import TypeVar, Generic, Optional, ClassVar, Any

class IntField:
    type = int

class PersonBase(type):
    def __new__(cls):
        for attr, value in cls.__dict__.items():
            if not isinstance(value, IntField):
                continue
            setattr(cls, attr, value.type())
        return cls

class Person(PersonBase):
    age = IntField()

person = Person()

print(type(Person.age)) # <class 'int'>
print(type(person.age)) # <class 'int'>
person.age = 25 # Incompatible types in assignment (expression has type "int", variable has type "IntField")

age 属性的类型将是 int 类型，但 MyPy 不能遵循该类型。

有没有办法让 MyPy 理解？

Django 已经实现了：

from django.db import models

class Person(models.Model):
    age = models.IntegerField()

person = Person()
print(type(Person.age)) # <class 'django.db.models.query_utils.DeferredAttribute'>
print(type(person.age)) # <class 'int'>
person.age = 25  # No error

Django 是如何做到这一点的？

Answer 1

Patrick Haugh 是对的，我正试图以错误的方式解决这个问题。描述符是要走的路：

from typing import TypeVar, Generic, Optional, ClassVar, Any, Type

FieldValueType = TypeVar('FieldValueType')


class Field(Generic[FieldValueType]):

    value_type: Type[FieldValueType]

    def __init__(self) -> None:
        self.value: FieldValueType = self.value_type()

    def __get__(self, obj, objtype) -> 'Field':
        print('Retrieving', self.__class__)
        return self

    def __set__(self, obj, value):
        print('Updating', self.__class__)
        self.value = value

    def to_string(self):
        return self.value

class StringField(Field[str]):
    value_type = str

class IntField(Field[int]):
    value_type = int

    def to_string(self):
        return str(self.value)


class Person:
    age = IntField()

person = Person()
person.age = 25
print(person.age.to_string())

MyPy 完全可以理解这一点。谢谢！

Answer 2

由于您在类上定义了字段，因此实用的方法是对字段进行类型提示。请注意，您必须告诉 mypy 不要检查线路本身。

class Person(PersonBase):
    age: int = IntField()  # type: ignore

这是最小的变化，但相当不灵活。

---

您可以使用带有假签名的辅助函数来创建自动键入的通用提示：

from typing import Type, TypeVar


T = TypeVar('T')


class __Field__:
    """The actual field specification"""
    def __init__(self, *args, **kwargs):
        self.args, self.kwargs = args, kwargs


def Field(tp: Type[T], *args, **kwargs) -> T:
    """Helper to fake the correct return type"""
    return __Field__(tp, *args, **kwargs)  # type: ignore


class Person:
    # Field takes arbitrary arguments
    # You can @overload Fields to have them checked as well
    age = Field(int, True, object())

这就是attrs 库提供其遗留提示的方式。这种风格允许隐藏注释的所有魔法/技巧。

---

由于元类可以检查注释，因此无需将类型存储在字段上。您可以将 Field 用于元数据，并为类型使用注释：

from typing import Any


class Field(Any):  # the (Any) part is only valid in a .pyi file!
    """Field description for Any type"""


class MetaPerson(type):
    """Metaclass that creates default class attributes based on fields"""
    def __new__(mcs, name, bases, namespace, **kwds):
        for name, value in namespace.copy().items():
            if isinstance(value, Field):
                # look up type from annotation
                field_type = namespace['__annotations__'][name]
                namespace[name] = field_type()
        return super().__new__(mcs, name, bases, namespace, **kwds)


class Person(metaclass=MetaPerson):
    age: int = Field()

这就是attrs 提供其 Python 3.6+ 属性的方式。它既通用又符合注释样式。请注意，这也可以用于常规基类而不是元类。

class BasePerson:
     def __init__(self):
         for name, value in type(self).__dict__.items():
             if isinstance(value, Field):
                 field_type = self.__annotations__[name]
                 setattr(self, name, field_type())


class Person(BasePerson):
    age: int = Field()