为什么此插入排序算法输出无？

Question

def insertion_sort(list):
    for index in range(1,len(list)):
        value = list[index]
        i = index - 1
        while i>=0:
            if value < list[i]:
                list[i+1] = list[i]
                list[i] = value
                i = i - 1
            else:
                break

a = [7,1,3,5,9,2,3]

print(insertion_sort(a))

This code is taken from a Khan Academy video . However, when I try to run it myself both on Jupyter Notebook and IDLE it outputs None. I can't figure out why when it's exactly the same from the video. Thanks in advance for helping.

Answer 1

Your function doesn't have a return statement anywhere. If a function doesn't return anything explicitly, it always returns None .

And it's almost certainly intentional that this function doesn't return anything.

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Notice that the function mutates its argument in-place:

>>> a = [7,1,3,5,9,2,3]
>>> insertion_sort(a)
>>> a
[1, 2, 3, 3, 5, 7, 9]

In Python, it's idiomatic for functions that do this to not return anything. See, for example, builtin methods like list.sort and list.append .

And notice that in the cited video , this is exactly what the instructor does—they don't print the result of insertion_sort , they just call it, and then look at the value of the variable.

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Also, in general, functions that mutate a value in-place, like this one, take an imperative verb, like the list method sort , while functions that leave the argument unchanged but return a transformed value, take a past participle, like the built-in function sorted .

So, if this function were named insertion_sorted , Python programmers would expect it to leave its argument alone and return a new, sorted list; since it's named insertion_sort , they'll expect it to sort its argument in-place and return nothing.