类方法返回实例的 MyPy 注释

Question

我应该如何将 @classmethod 注释为 returns cls 的实例？这是一个不好的例子：

class Foo(object):
    def __init__(self, bar: str):
        self.bar = bar

    @classmethod
    def with_stuff_appended(cls, bar: str) -> ???:
        return cls(bar + "stuff")

This returns a Foo 但更准确地说 returns 调用 Foo 的哪个子类，所以用 -> "Foo" 注释不会够用了。

Answer 1

诀窍是明确地向 cls 参数添加注释，结合 TypeVar，对于 generics, and Type, to represent a class rather than the instance itself，像这样：

from typing import TypeVar, Type

# Create a generic variable that can be 'Parent', or any subclass.
T = TypeVar('T', bound='Parent')

class Parent:
    def __init__(self, bar: str) -> None:
        self.bar = bar

    @classmethod
    def with_stuff_appended(cls: Type[T], bar: str) -> T:
        # We annotate 'cls' with a typevar so that we can
        # type our return type more precisely
        return cls(bar + "stuff")

class Child(Parent):
    # If you're going to redefine __init__, make sure it
    # has a signature that's compatible with the Parent's __init__,
    # since mypy currently doesn't check for that.

    def child_only(self) -> int:
        return 3

# Mypy correctly infers that p is of type 'Parent',
# and c is of type 'Child'.
p = Parent.with_stuff_appended("10")
c = Child.with_stuff_appended("20")

# We can verify this ourself by using the special 'reveal_type'
# function. Be sure to delete these lines before running your
# code -- this function is something only mypy understands
# (it's meant to help with debugging your types).
reveal_type(p)  # Revealed type is 'test.Parent*'
reveal_type(c)  # Revealed type is 'test.Child*'

# So, these all typecheck
print(p.bar)
print(c.bar)
print(c.child_only())

一般情况下，cls（和self）可以不加注解，但如果需要引用具体的子类，可以加一个explicit annotation。请注意，此功能仍处于试验阶段，在某些情况下可能存在错误。您可能还需要使用从 Github 克隆的最新版本的 mypy，而不是 pypi 上可用的版本——我不记得该版本是否支持类方法的此功能。

Answer 2

只是为了完整性，在 Python 3.7 中，您可以通过在文件开头导入 from __future__ import annotations 来使用 PEP 563 中定义的 postponed evaluation of annotations。

然后你的代码看起来像

from __future__ import annotations

class Foo(object):
    def __init__(self, bar: str):
        self.bar = bar

    @classmethod
    def with_stuff_appended(cls, bar: str) -> Foo:
        return cls(bar + "stuff")

根据 docs，此导入将从 Python 3.11.

有效地自动开始

类方法返回实例的 MyPy 注释

MyPy annotation for classmethod returning instance

python

mypy