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Python Python Collections (2016, retired 2019) Slices Introduction To Slices

olegovich7
olegovich7
6,605 Points
Posted by olegovich7
olegovich7
6,605 Points

List sorting

I can't really understand: if execute this in python shell:

>>>a = [5,4,3,2,1] 
>>>b = a
>>>b.sort()
>>> b                                                                                                                                                
[1, 2, 3, 4, 5]                                                                                                                                      
>>> a                                                                                                                                                
[1, 2, 3, 4, 5]

why a is also sorted?

3 Answers

Umesh Ravji
Umesh Ravji
42,386 Points
Umesh Ravji
Umesh Ravji
42,386 Points

Hi olegovich7, that's because a and b are the same list. When you assign a to b, you aren't making a copy of the list, you are just making it point to the same place in memory. Performing an operation on either variable performs that operation on the single list in memory.

Instead you could make a copy, see the code below.

a = [5,4,3,2,1]
b = a[:]
b.sort()
print(a)  # [5, 4, 3, 2, 1], sorting b has no effect on a
andren
andren
28,558 Points
andren
andren
28,558 Points

Because this line:

b = a

Makes b and a contain the same list. Not just the same list content, but literally the same list.

In Python when you assign a list to a variable you in reality assign it a reference to a point in memory where the list is stored. When you set another variable equal to that list what actually gets set to the variable is not the contents of the list but the reference to the list. Meaning that both variables now point to the same location in memory.

This means that in reality while there are two variables pointing at the list there is only one actual list existing. So any modifications to that list, be it through variable a or variable b will end up changing that list for both of them.

This is a pretty standard behavior you will find in a lot of languages, certain data types will pass a memory reference when you use them and some will pass the value they store.

So far you have mostly worked with value-based types like string and int. It's worth mentioning that some of this is a bit simplified but it will hopefully get you to somewhat understand what is going on.

NARCIS MATABOSCH
NARCIS MATABOSCH
1,540 Points
NARCIS MATABOSCH
NARCIS MATABOSCH
1,540 Points

Thank you very much for these posts because I didn't understand the use of [:] to copy a list =)