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So, I need to use a for i in loop on a list that is within a list. Let me give you an example.
I have a blacklist (list) that contains 10 categories (also lists), in those categories, there are always 2 ints.
I also have a list called x. x contains 2 ints.
I want to check if the 2 ints inside x are the same as any of the blacklist categories.
In order to do that I need to do

def blacklist_check(blacklist, z):
    for i in blacklist:
        for y in blacklist[i]:
            difference = blacklist[i][y] - z
            if difference < 10 and difference > -10:
                print(f"{difference} found")
                return False
            print(f"{difference} not found")
            if i == 10:
                return True

but when I try that I get

TypeError: list indices must be integers or slices, not list

I can not transfer the categories to ints or any other type than lists. How do I make this work?

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  • 2
    Just for y in i and difference = y - z. Note that i is the element in the blacklist, not their index. Commented Jul 4, 2022 at 3:25
  • @MechanicPig Please post this as an answer. Commented Jul 4, 2022 at 3:42
  • @MeLikeFish First debug your program to see what is going on. This article has some great tips to get you started. Then if you still need help post a new quesiton with the current version of your code. Be sure to explicitly explain what "it always returns None" means. Usually with some example output. Commented Jul 4, 2022 at 4:28
  • @Code-Apprentice I found the issue, wanted to edit the comment but it didn't let me. Thank you for the reply though Commented Jul 4, 2022 at 4:37
  • @MeLikeFish Thanks for letting me know. You can only edit a comment within a few minutes of posting it. But you can and should delete comments if they are no longer relevant or helpful. Good luck. Commented Jul 4, 2022 at 7:53

3 Answers 3

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Here (y) is a list, you cannot nest a list as a index inside another list. Try this

def blacklist_check(blacklist, z):
    for i in blacklist:
        for y in range(len(i)):
            difference = i[y] - z
            if difference < 10 and difference > -10:
                print(f"{difference} found")
                return False
            print(f"{difference} not found")
            if i == 10:
                return True
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3 Comments

I think you mean that i is a list, not (y). Also i[y] should change to y.
Yes, also can use (range and len) function Like the example above
Yes, that works, too. In Python, we prefer for x in my_list as much as possible and don't use range() very much.
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Python handles for loops differently to perhaps a more conventional C-Type language in that it runs based on enumeration rather than incrementation.

In your case, what you have with for i in blacklist, for each iteration of the for loop, i is set to the element rather than the index, so if your data structure looks something like:

[ [1,2,3,4], [5,6,7,8] ]

Then the first value of i will be [1, 2, 3, 4], and is hence a list. If you want the indexes rather than the elements, you could use the range and len functions in the following way:

for i in range(len(blacklist)):

which will make i be 0 on the first iteration, and 1 on the second one, etc.

That would mean that calling blacklist[i] will return [1, 2, 3, 4]

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If you don't consider the printed statements in the sample code, the simplest way to write it should be to use any with generator expression:

def blacklist_check(blacklist, z):
    return not any(abs(y - z) < 10 for cls in blacklist for y in cls)

This is equivalent to:

def blacklist_check(blacklist, z):
    for cls in blacklist:
        for y in cls:
            if abs(y - z) < 10:   # same as -10 < (y - z) < 10
                return False
    return True

Note that since i in your code does not represent the index of the element in the blacklist, you should not judge whether it is the last element by i == 10, but should directly return True after the end of the for loop.

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