Sorting lists in python issue

Question

I started learning python few days ago (with no prior programming experience nor knowledge) and am currently stuck with the following thing I do not understand: Let' say I have an unsorted list "b" and I want to sort a list "c" which looks exactly like list "b":

b = [4,3,1,2]
c=b
c.sort()

print b
print c

What I discovered is that both b and c are sorted: [1,2,3,4] [1,2,3,4]

Why is that so?

It seems this solution works perfectly when I create a copy of the "b" list:

b = [4,3,1,2]
c=b[:]
c.sort()

print b
print c

Results in: [4,3,1,2] [1,2,3,4]

But why does the first solution not work?

Thank you.

Answer 1

You already seem to understand that c = b is different to c = b[:] . In the first case c references the same object as b . In the latter it references a copy of b .

So it shouldn't be surprising that since b.sort() sorts the list referenced by b , When you inspect c it is also sorted - because it's the same object

The usual way to decouple a sorted list from the original is

c = sorted(b)

Answer 2

Because in the first solution, b and c both point to the same object. The slicing in the second solution creates a new object with the same contents as the old.

Answer 3

In the first sample, you are copying b into c by reference, which means that whenever any change (sorting, for example) is made on b , it will be applied on c , because basically they both point to the same object.

In the second sample, you are copying the array by value , not by reference , which create an entirely new object in the memory. Therefore, any changes made on the one of them will not be applied on the other one.