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?
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.