Question

I have the following code and I’m wondering why it returns the same list instead of a copy:

x = [2,1,3]
y = x
y.sort()
print y
print x

Why does this return the same sorted list?

Answer 1

Because it’s modified in-place.

Python lists have reference semantics, meaning that when you assign a list to another variable, they’re actually pointing at the same list.

If you want to make a copy, do this:

x = [2,1,3]
y = x[:]
y.sort()
print y
print x

The slice notation does cause the list (in this case, the entire list, though you can make a small modification to ask for a particular sublist) to be copied.