python - 我可以使用 python 装饰器根据输入类型预处理输入和后处理输出吗？

Question

我有可以接受 List 或 str 的函数。返回类型应与输入类型匹配。例如，如果给定一个 str，该函数应该返回一个 str。

这是一个说明这一点的玩具示例:

def swap_first_and_last(a: Union[List, str]) -> Union[List, str]:
    # Not in-place.
    STR = isinstance(a, str)
    a = list(a)

    a[0], a[-1] = a[-1], a[0]

    return "".join(a) if STR else a

这是一个实际的例子:

def next_permutation(a: Union[List, str]) -> Union[List, str]:
    """
    Not in-place.
    Returns `None` if there is no next permutation
    (i.e. if `a` is the last permutation).
    The type of the output is the same as the type of the input
    (i.e. str input -> str output).
    """
    STR = isinstance(a, str)
    a = list(a)

    N = len(a)
    i = next((i for i in reversed(range(N-1)) if a[i] < a[i + 1]), None)
    if i is None:
        return None
    j = next(j for j in reversed(range(i+1, N)) if a[j] >= a[i])

    a[i], a[j] = a[j], a[i]
    a[i + 1:] = reversed(a[i + 1:])

    return "".join(a) if STR else a

如您所见，只有几行专门用于处理 str 输入与 List 输入，即:

    # preprocess
    STR = isinstance(a, str)
    a = list(a)
    
    # main logic
    ...
    
    # postprocess
    return "".join(a) if STR else a

我可以使用装饰器来进行这种轻微的预处理和后处理吗？

Answer 1

是的，你可以使用这样的装饰器:

from typing import Union, List

def pre_and_post_processing(func):
    def inner(a: Union[List, str]) -> Union[List, str]:
        STR = isinstance(a, str)
        a = list(a)
        b = func(a)
        return "".join(b) if STR else b
    return inner   


@pre_and_post_processing
def swap_first_and_last(a: List) -> List:
    a[0], a[-1] = a[-1], a[0]
    return a


print(swap_first_and_last("asd"))  # -> dsa
print(swap_first_and_last(["asd", "ds", "sds"]))  # -> ['sds', 'ds', 'asd']

请注意，swap_first_and_last 函数现在获取并返回一个List。