久々にcheckioをやってみて、sumを使用しないでリスト内の要素を足して行く問題があった。
自分のコードができない理由が不明のため、メモとして保存しておく。

下記問題文

Our new calculator is censored and as such it does not accept certain words. You should try to trick by writing a program to calculate the sum of numbers.

Given a list of numbers, you should find the sum of these numbers. Your solution should not contain any of the banned words, even as a part of another word.

The list of banned words are as follows:

sum
import
for
while
reduce
Input: A list of numbers.

Output: The sum of numbers.

回答コード

scal = 0

def checkio(data):
    global scal
    if len(data) == 0:
        return scal
    else:
        scal += data[0]
        del data[0]
        return checkio(data)