Why do I need to loop twice to find all of the items I'm looking for?

Question

I am working my way through the Learn Python course. I just completed the manipulating lists code challenge in the Lists Redux section. I was able to make it work but I don't understand why I had to take the approach that I took.

In this challenge you are presented with a list, the_list = ["a", 2, 3, 1, False, [1, 2, 3]], and then you have to use .remove() and/or del to remove all string, boolean, and list members from the_list.

In my initial approach I iterated over the list with a for loop and used isinstance() to test each item. I found that I could remove booleans and strings this way but I could not remove the list within the_list. This was really strange because some debugging that I did with print() showed that I was detecting the list within the_list.

After a lot of trial and error I created a second for loop which executes after the first one is complete. In this second for loop I once again used isinstance() to check for the list within the_list and this time I was able to remove the list within the_list.

So, I've passed the challenge but I don't see why I couldn't do this with one loop instead of two.

Here is the snippet of code that I tried initially. I don't understand why this does not work.

for item in the_list:
    if isinstance(item, (str, bool, list)):
        the_list.remove(item)

Answer 1 · 2014-12-02T17:23:04Z

December 2, 2014 5:23pm

Hello my friend.

I've checked your code, the returning list is the following:

[2, 3, 1, [1,2,3]]

I recommend you to not remove items on the same list you're iterating, always iterate on a copy of itself and inside the loop remove the items of the original list. Here's what I'm saying:

for item in list(the_list):
    if isinstance(item, (str, bool, list)):
        the_list.remove(item)

Using list(the_list) makes a copy of the original "the_list", with that you can assure that it will loop through the entire list.

Hope this works :)

Answer 2 · 2014-12-02T17:44:58Z

December 2, 2014 5:44pm

You, technically, don't have to use any loops for this :)

the_list.remove(False)
the_list.remove([1, 2, 3])
the_list.remove("a")

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Why do I need to loop twice to find all of the items I'm looking for?

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