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Python Python Basics (2015) Python Data Types list.remove()

traolach trewhella
PLUS
traolach trewhella
Courses Plus Student 835 Points
Posted by traolach trewhella
traolach trewhella
Courses Plus Student 835 Points

okay for this 2 /part i dont understand the list of colors what is a list of colors and how do i remove it please help

i tried copy the colors it didint work how do i remove colors and agian what are colors?

lists.py 
states = [
    'ACTIVE',
    ['red', 'green', 'blue'],
    'CANCELLED',
    'FINISHED',
    5,
]
states.remove(5)

2 Answers

Alex Sell
Alex Sell
19,355 Points
Alex Sell
Alex Sell
19,355 Points

Hi Traolach,

So a list in python is basically just a collection or grouping of items or elements. For example [1, 2, 3] is a list of numbers. ["dog", "cat", "mule"] is a list of animals. ["dog", 2, "s"] is a list of miscellaneous items. You can even have empty lists: []. In other programming languages, lists are often referred to as arrays.

To answer your question, ['red', 'green', 'blue'] would be the list of colors you are looking for. In fact, you can see that ['red', 'green', 'blue'] is a list that is inside another list called states.

When you call the remove method on states, you have to pass in the object you wish to remove from the list. So if I pass in the string 'ACTIVE', I am removing 'ACTIVE' from the states list. e.g. states.remove('ACTIVE')

If I want to remove the list ['red', 'green', 'blue '] from the states list, I would have to do something very similar.

Hope this helps!

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,426 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,426 Points

You can remove the list in two ways:

states = [
    'ACTIVE',
    ['red', 'green', 'blue'],
    'CANCELLED',
    'FINISHED',
    5,
]
states.remove(5)

# remove using explicit list
states.remove(['red', 'green', 'blue'])

# OR using the index of the list within 'states'. The list is at index 1
states.remove(states[1])
# but this is the same as 
states.pop(1)
# or as
del states[1]
Ge Ode
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.a{fill-rule:evenodd;}techdegree
Ge Ode
Full Stack JavaScript Techdegree Student 9,002 Points
Ge Ode
.a{fill-rule:evenodd;}techdegree
Ge Ode
Full Stack JavaScript Techdegree Student 9,002 Points

Hey Chris, follow up question for you -

I gather that you can only remove 1 value at a time using the list.remove method, but why? I imagine that if we could input multiple values in a list.remove method, the list would have to be reindexed for each value removed. But is there a reason why loops (or methods i don't know) do this instead?

Thanks!

Chris Freeman
Chris Freeman
Treehouse Moderator 68,426 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,426 Points

There is an inherent danger of looping over a list while also adding or removing elements due to the reindex (or mis-indexing) that occurs. Working around this usually means making a copy of the list to loop over but still make changes to the original list. I'm unaware of a builtin construct that would notice that an element of the iterable had been deleted and would then automatically skip to the correct next element.

Sometimes the limitations of a method, like remove(), have to do with the resultant behavior instead of the input limitation.

For list.remove() it raises an error if the item is not found:

list.remove(x)
Remove the first item from the list whose value is x. It is an error if there is no such item.

So if more than one removal item was provided as an argument, how should the method behave if first argument was present and removed but the second element wasn't?

What of the first argument was not present but the second argument was. Does the get error get raised before the second argument is removed?

Basically it best to keep the methods simple then wrap them in loops or list comprehensions to get the extended functionality.

If the removal is extensive then maybe conversion to sets may be called for so the set.difference() method could be used.

Another consideration is whether you want to keep the original list intact and produce a new sublist or whether the original list needs to changed.