Why b also change?Please explain me!

Question

a = [[1,2],[3,4]]
b = a.copy()
a[0].reverse()
print(a)
print(b)
Output>>[[2,1], [3,4]]
>>[[2,1], [3,4]]

Answer 1

Because you have made a shallow copy instead of a deep copy.

You made a new outer list, but it contains references to the same inner lists.

More details:

https://code.sololearn.com/c89ejW97QsTN/?ref=app

Answer 2

If you want to make a deep copy of that list a, one quick way to do it is:

b = [l[:] for l in a]

Answer 3

HonFu Thank you very much

Answer 4

These things are confusing in the beginning, but I am sure you'll understand it much better after studying my tutorial up there.

This is a topic that will come up again and again, so the effort will definitely not be wasted. :)

Answer 5

I am reading your explanations now.But I am wondering What am I supposed to do now?

Answer 6

What do you mean? Supposed to do with what? What do you *want* to do?

Answer 7

If I change a[0] ,
I want b[0] remains unchanged.

Answer 8

HonFu Sorry my English is not
very good.
When I replace third line with
a[0] = a[0][::-1] , b remains unchanged. What is the difference?
a = [[1,2],[3,4]]
b = a.copy()
a[0] = a[0][::-1]
print(a)
print(b)
Output>>[[2,1], [3,4]]
>>[[1,2], [3,4]]

Answer 9

Because it is list. Direct equal do not create copy. Make slicing of list. In this case new copied list created