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)
4324146424
```