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Python

DaJuan Harris
PLUS
DaJuan Harris
Courses Plus Student 983 Points
Posted by DaJuan Harris
DaJuan Harris
Courses Plus Student 983 Points

Why does insert(-1) insert the item at the beginning of the list.

I would think that the following code: 

    try:
        #Position is now the absolute integer of itself
        position = abs(int(position))
    except ValueError:
        position = None
    if position is not None:
        shopping_list.insert(position-1, item)

Would put the item at the end of the list if position was set to 0, because it would make position now be -1. It does what I want it do, but why is what I want to know.

Edit: I found out that if there is more than two items in the list, it doesn't put it at the beginning, but instead it will put it in front of the last item in the list. This just seems like a weird python oddity

1 Answer

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

An insert index of -1 will always put the new item just before the last item. That's because "-1" represents "the last item", and the insert function puts the new item before the one listed in the index.

DaJuan Harris
DaJuan Harris
Courses Plus Student 983 Points
DaJuan Harris
DaJuan Harris
Courses Plus Student 983 Points

Yeah the user is choosing where to put the item in the list. It's being subtracted by 1, because of how indexing works. That's so if a user doesn't know how indexing works, they won't break the program. However, I do know how it works, so I was messing around with it. Below is the full code for the function

def add_to_list(item):
    show_list()
    if shopping_list:
        position = input(f"""Where should I add {item}?\n
                        Press Enter to add to the end of list\n
                        > """)
    else:
        position = 0
    try:
        #Position is now the absolute integer of itself
        position = abs(int(position))#
    except ValueError:
        position = None
    if position is not None:
        shopping_list.insert(position-1, item)
    else:
        shopping_list.append(item)

    show_list()
Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

So it seems a bit odd, but the function is actually performing consistently. The only way to cause "insert" to place an item at the end of a list is to use an index higher than the last item (such as the length of the list).