Python references

Question

Can someone explain why the example with integers results in different values for x and y and the example with the list results in x and y being the same object?

x = 42
y = x
x = x + 1
print x # 43
print y # 42

x = [ 1, 2, 3 ]
y = x
x[0] = 4
print x # [4, 2, 3]
print y # [4, 2, 3]
x is y # True

Answer 1

The best explanation I ever read is here: http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables

Answer 2

Because integers are immutable, while list are mutable. You can see from the syntax. In x = x + 1 you are actually assigning a new value to x (it is alone on the LHS). In x[0] = 4, you're calling the index operator on the list and giving it a parameter - it's actually equivalent to x.__setitem__(0, 4), which is obviously changing the original object, not creating a new one.