Tic Tac Toe - Invent Your Own Computer Games with Python - page 163

10 - Tic Tac Toe
>>> spam = [0, 1, 2, 3, 4, 5]
>>> cheese = spam
>>> cheese[1] = 'Hello!'
>>> spam
[0, 'Hello!', 2, 3, 4, 5]
>>> cheese
[0, 'Hello!', 2, 3, 4, 5]
Notice that the line cheese = spam copies the list reference in spam to cheese ,
instead of copying the list value itself. This is because the value stored in the spam
variable is a list reference , and not the list value itself. This means that the values stored in
both spam and cheese refer to the same list. There is only one list because the list was
not copied, the reference to the list was copied. So when you modify cheese in the
cheese[1] = 'Hello!' line, you are modifying the same list that spam refers to.
This is why spam seems to have the same list value that cheese does.
Remember when you first learned about variables, I said that variables were like boxes
that contain values. List variables don't actually contain lists at all, they contain references
to lists. Here are some pictures that explain what happens in the code you just typed in:
Figure 10-5: Variables do no store lists, but rather references to lists.
On the first line, the actual list is not contained in the spam variable but a reference to
the list. The list itself is not stored in any variable.