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
That's because when you have a list or a tuple in python you create a reference to an object. When you say that y = x you reference to the same object with y as x does. So when you edit the object of x y changes with it.
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.
The best explanation I ever read is here: http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables
If you do y = x, y and x are the reference to the same object. But integers are immutable and when you do x + 1, the new integer is created:
>>> x = 1
>>> id(x)
135720760
>>> x += 1
>>> id(x)
135720748
>>> x -= 1
>>> id(x)
135720760
When you have a mutable object (e.g. list, classes defined by yourself), x is changed whenever y is changed, because they point to a single object.
As the previous answers said the code you wrote assigns the same object to different names such aliases. If you want to assign a copy of the original list to the new variable (object actually) use this solution:
>>> x=[1,2,3]
>>> y=x[:] #this makes a new list
>>> x
[1, 2, 3]
>>> y
[1, 2, 3]
>>> x[0]=4
>>> x
[4, 2, 3]
>>> y
[1, 2, 3]