使用函数的返回类型指定类型提示

我想将变量的类型暗示为特定函数的返回类型（无需手动指定函数的返回类型是什么）。

我无法以可用作另一个变量提示的方式从函数中提取返回值类型。

def dostuff() -> T:
    StuffContainer = namedtuple("StuffContainer", ["x", "y"])
    return StuffContainer(1, 2)

def getX(a : T):
    return a.x

C++ 相当于我想要做的事情：

auto dostuff() {
    struct data {
        int x;
        int y;
    };
    return data {1, 2};
}

int getX(decltype(dostuff()) a) {
    return a.x;
}

没有等同于decltype在 PEP 484 打字生态系统中。更广泛地说，并没有真正表达“匿名”类型的好方法。

因此，键入代码的规范方式是执行类似以下操作：

StuffContainer = namedtuple("StuffContainer", ["x", "y"])

def dostuff() -> StuffContainer:
    return StuffContainer(1, 2)

def getX(a: StuffContainer):
    return a.x

如果担心您返回的类型太长并且不方便写出，您也许可以使用以下命令将其缩短一点类型别名 https://docs.python.org/3/library/typing.html#type-aliases:

StuffContainer = namedtuple("StuffContainer", ["x", "y"])

# S is now an alias of StuffContainer. The two types can be
# used interchangeably.
S = StuffContainer

def dostuff() -> S:
    return StuffContainer(1, 2)

def getX(a: S):
    return a.x

如果担心的是您不想对其进行编码dostuff回报具体来说一个命名元组，并且您只想承诺返回带有“x”和“y”属性的某个对象，您也许可以使用协议——您可以找到有关它们的更多信息在政治公众教育中 https://www.python.org/dev/peps/pep-0544/并在mypy 文档 https://mypy.readthedocs.io/en/stable/protocols.html。 （不幸的是，尽管官方 Python 类型模块文档中还没有任何关于它们的信息。）

例如：

from typing import NamedTuple

# If you're using Python 3.8+, you can do:
from typing import Protocol

# If you want to support older versions of Python,
# run 'pip install typing_extensions' and do the below instead
from typing_extensions import Protocol

# Any type that has 'x' and 'y' attributes is compatible with this.
class HasXAndY(Protocol):
    # Making these properties to declare that they're read-only,
    # for maximum flexibility.
    @property
    def x(self) -> int: ...
    @property
    def y(self) -> int: ...


def dostuff() -> HasXAndY:
    # Note: I'm switching to a version of namedtuple that lets me associate
    # types with each field, mostly just for demonstration purposes. At runtime,
    # it behaves identically to collections.namedtuple.
    StuffContainer = NamedTuple("StuffContainer", [("x", int), ("y", int)])
    return StuffContainer(1, 2)

def get_x(obj: HasXAndY) -> int:
    return obj.x

# Type-checks
get_x(dostuff())


class Unrelated:
    def __init__(self, x: int, y: int, z: int) -> None:
        self.x = x
        self.y = y
        self.z = z

# Also type-checks
get_x(Unrelated(1, 2, 3))