Can I use a python decorator to preprocess input and postprocess output based on an input type?

user_13881506 0
Viewed 187

I have functions that can take either a List or a str. The return type should match the input type. E.g., if given a str the function should return a str.

This is a toy example to illustrate the point:

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

This is an actual example:

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

As you can see, only a few lines are dedicated to handling str input vs List input, namely:

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

Can I use a decorator to do this slight preprocessing and postprocessing?

typing overloading decorator python
2 Answers

Yes, you can use a decorator like this:

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']

Note that the swap_first_and_last function now get and return a List.

user_89027820

Following up on @larsks suggestion, here is a way to use singledispatch on the @joseville toy example:

Example:

from functools import singledispatch

@singledispatch
def swap_first_and_last(a: str) -> str:

    a = list(a)
    swap_first_and_last(a)
    return "".join(a)

@swap_first_and_last.register
def _(a: list) -> list:
    
    a[0], a[-1] = a[-1], a[0]
    return a

print(swap_first_and_last([1,2,3]))
print(swap_first_and_last("123"))

Output:

[3, 2, 1]
321
user_86355470
Related