Function Returning None While Operation Returning Value

Question

I have written a function that reverses the values of a string:

def reversed_list(in_list):
    """A function that reverses the order 
    of the items in a list"""
    
    flipped_list = []
    
    [flipped_list.insert(0, in_list[k]) for k, v in enumerate(in_list)]
        
    return flipped_list
    
reversed_list([1, 2, 3, 4, 5, 6])

It works as expected. However, if I combine the last two lines of the function to become:

return [flipped_list.insert(0, in_list[k]) for k, v in enumerate(in_list)]

The outputted list is [None, None, None...]

Why is this and how can I combine these last two lines to make this code as elegant as possible?

Cheers,

Why are you using a list comprehension here? A simple for loop would be the correct choice — UnholySheep, Sep 10 '20 at 17:36
You don't need `enumerate`, either. The only reason you need `k` is to compute `in_list[k]`, but `v` is already `in_list[k]`. `flipped_list.insert(0, v) for v in in_list` would suffice. — chepner, Sep 10 '20 at 17:42
And there's little reason trying to find something more elegant than `return list(reversed(in_list))`. — chepner, Sep 10 '20 at 17:43
@chepner: Or just `return in_list[::-1]` if the input is expected to be a `list`, or returning the same type as the input is fine. — ShadowRanger, Sep 10 '20 at 17:49
@chepner, I am just playing around and would normally use `reverse()`. Basically, I am going through Python functions and trying to rewrite them as elegantly as possible from scratch. TBH, I don't want to be using `.insert()` either. Thanks for the trimming in your first response though — GalacticPonderer, Sep 10 '20 at 17:53
@UnholySheep - Correct me if I am wrong, but are list comprehensions just elegant one-line loops? — GalacticPonderer, Sep 10 '20 at 17:54
Does this answer your question? [Python's insert returning None?](https://stackoverflow.com/questions/1516889/pythons-insert-returning-none) — Georgy, Sep 10 '20 at 17:56
*"are list comprehensions just elegant one-line loops"* - No, list comprehensions are dedicated syntax to create lists filled with calculated elements (and optimized in the interpreter itself). Using it when you have no intention of using the created list is just a waste (allocating a list object that is immediately discarded) — UnholySheep, Sep 10 '20 at 18:03
Fair point. However, I am returning the list, then using it outside the function. So, is your point applicable in this situation? — GalacticPonderer, Sep 10 '20 at 18:36

score 3 · Answer 1 · answered Sep 10 '20 at 17:38

flipped_list.insert modifies the list in place, and returns None.

If the first example, you are updating flipped_list within the list comprehension and then are returning the modified list.

In the second example, you are still modifying flipped_list, but returning the list that was created in the list comprehension which is filled with None values

Function Returning None While Operation Returning Value

1 Answers1