> This example does NOT contradict tuple immutability. You assume that +=
> operates in place, while it is not. You can see a += in Python just as a = a +.
> To confirm:
> >>> a = (1,2)
> >>> b = a
> >>> print id(a)
> 4324146424
> >>> print id(b)
> 4324146424
> >>> a+=(3,)
> >>> a
> (1, 2, 3)
> >>> b
> (1, 2)
> >>> print id(a)
> 4324211440
> >>> print id(b)

+1 :-)