And, == operators in the same python if statement

Question

Given a list of integers:

old_list = [1,1,2,-2,5,2,4,4,-1,-2,5]

I run the following code to get all the integers grouped in a list of lists:

old_list.sort()
    new_list = []
    for i in old_list:
        if new_list == []:
            new_list.append([i])
        elif new_list[-1][0] == i:
            new_list[-1].append(i)
        else:
            new_list.append([i])
    print(new_list)

Returning my desired output:

[[-2, -2], [-1], [1, 1], [2, 2], [4, 4], [5, 5]]

On the one hand I found I could condense the if elif statement using and, while retrieving the same output:

old_list.sort()
    new_list = []
    for i in old_list:
        if new_list and new_list[-1][0] == i:
            new_list[-1].append(i)
        else:
            new_list.append([i])
    print(new_list)

On the other hand if I try to remove the new_list and from the if statement the following error arises:

IndexError: list index out of range

Why isn't the if new_list and new_list[-1][0] == i: statement firing the same index error?

bipll · Accepted Answer · 2020-05-21 13:19:27Z

1

Because first you check that new_list is not empty, so at least it has a last element. When you remove this check, since your new_list is initially empty, new_list[-1] is out of range.

answered May 21, 2020 at 13:19

bipll

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1 Comment

Tomerikoo Over a year ago

@pyn00b that's called short-circuiting. If new_list is empty, the other part of the and will not even be checked and the whole condition is false. On the other hand, if it is true, you know that there is at least one element...

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