Python Simple Swap Function

I came across this problem when attempting to learn python. Consider the following function:

def swap0(s1, s2):
    assert type(s1) == list and type(s2) == list
    tmp = s1[:]
    s1 = s2[:]
    s2 = tmp
return

s1 = [1]
s2 = [2]
swap0(s1, s2)
print s1, s2

What will s1 and s2 print?

After running the problem, I found that the print statement will print 1 2. It seems that the value of s1 and s2 did not change from the swap0 function. The only explanation that I could think of was because of the line.

tmp = s1[:]

Since s1[:] is a copy, this makes sense that the value of s1 will not change in the function call. However because the parameter of swap0 is (s1, s2), I am not sure if after doing tmp = s1[:]. Anytime I do

s1 = something...

it will be a reference to the copy of s1, instead of s1 itself. Can someone offer a better explanation? Thanks.