Unexpected change to a list in a small amount of self-contained code

[Johno]

I am using Python 3. I am getting an unexpected result: a list is changing, but I do not understand how/where it changes. Here is my code:

def cool(goodlist, letter):
  goodlist.append("foobar")
  goodlist.append(letter)
  newlist = goodlist
  return newlist

alista = []

var1 = cool(alista, "J")
print(var1)
print(alista)

The above program prints out this:

Output:
['foobar', 'J']
['foobar', 'J'][/font]

I would expect alista to be empty still. Why and what modifies alista?