+= python
In [1]: x = [2, 3, 4]
In [2]: y = x
In [3]: x += 7, 8, 9
In [4]: x
Out[4]: [2, 3, 4, 7, 8, 9]
In [5]: y
Out[5]: [2, 3, 4, 7, 8, 9]
In [6]: x += [44, 55]
In [7]: x
Out[7]: [2, 3, 4, 7, 8, 9, 44, 55]
In [8]: y
Out[8]: [2, 3, 4, 7, 8, 9, 44, 55]
In [9]: x = x + [33, 22]
In [10]: x
Out[10]: [2, 3, 4, 7, 8, 9, 44, 55, 33, 22]
In [11]: y
Out[11]: [2, 3, 4, 7, 8, 9, 44, 55]