如何输入注释reduce函数？

Question

我正在

list

 之上编写一个非常薄的包装器，我想定义一个名为 

reduce

 的方法，但我很难正确注释它，以便 

pylance

、

mypy

 和 

pylint

 剪切每当我使用该方法，甚至定义它时，他们都会抱怨。

我很不安地意识到几乎所有 Python 内置库都没有类型注释。

无论如何，这是我的实现，

    def reduce(self, func: Callable[[list[T], list[T]], list[T]] = lambda x, y: x + y, default: Optional[T] = None) -> 'List[T]':  # type: ignore
        from functools import reduce
        if default is None: return List(reduce(func, self.list))  # type: ignore
        return List(reduce(func, self.list, default))  # type: ignore

当我的

List

 实际上是字符串列表时，这会失败

a: List[str] = List(['a', 'b'])
b = a.reduce(lambda x, y: x + y)

显然在这里，类型检查器和 linter 说他们期望

list[T]

，而我通过了 

str

。

Answer 1

我认为在这种情况下，人们会使用

泛型类型，否则它无法完全推理你的类型。下面是一个使用支持reduce的通用类型的列表示例（因为我们强制它有一个 __add__

 方法）：

from functools import reduce
from typing import Sequence, TypeVar, Callable, Generic, Dict, Protocol, cast

A = TypeVar("A")


# define that our type supports the add operator, since it needs to be able to to reduce it
class SupportsAdd(Protocol[A]):
    def __add__(self, __other: A) -> A:
        ...


T = TypeVar("T", bound=SupportsAdd)


def _reduce_default(x: T, y: T) -> T:
    return x + y


# instead of using None as the default, we use a special object
# so that we can distinguish between None and a missing default
#
# (the underlying reduce function does the same thing)
_missing_default = object()


class MyList(Generic[T]):
    def __init__(self, initialdata: Sequence[T]) -> None:
        self.data = list(initialdata)

    def __getitem__(self, i: int) -> T:
        return self.data[i]

    def __setitem__(self, i: int, v: T) -> None:
        self.data[i] = v

    def __len__(self) -> int:
        return len(self.data)

    def __delitem__(self, i: int) -> None:
        del self.data[i]

    def reduce(
        self,
        func: Callable[[T, T], T] = _reduce_default,
        default: T = cast(T, _missing_default),
    ) -> T:
        if default is _missing_default:
            return reduce(_reduce_default, self.data)
        else:
            return reduce(func, self.data, default)

    # just another method that uses the types
    def add_keys(self, keys: Dict[T, int]) -> None:
        for k in keys:
            self.data.append(k)

它的一些用法：

def test() -> None:
    ll: MyList[int] = MyList([1, 2, 3])

    print(ll[1])

    ll[1] = 5

    print(ll[1])

    # add some keys
    ll.add_keys({30: 1, 50: 2, 60: 3})

    print(len(ll))

    val: int  # define the type separately, mypy warns us if it doesnt match
    val = ll.reduce()

    print(val)

    # using strings instead
    ll2: MyList[str] = MyList(["a", "b", "c"])

    print(ll2.reduce())

2
5
6
149
abc

如果您尝试使用某些不支持

__add__

的类，

mypy

会警告您：

# test class to see if classes without __add__ fail
class NoAdd:
    def __init__(self, v: int) -> None:
        self.v = v

x: MyList[NoAdd] = [NoAdd(1)]

test.py:100: error: Type argument "NoAdd" of "MyList" must be a subtype of "SupportsAdd[Any]"  [type-var]
test.py:100: error: Value of type variable "T" of "MyList" cannot be "NoAdd"  [type-var]
Found 2 errors in 1 file (checked 1 source file)