I'm trying:

```
>>> a = []
>>> id(a)
42952968L
>>> a.append(1)
>>> a
[1]
>>>id(a)
42952968L
>>> a += [2]
>>> a
[1, 2]
>>> id(a)
42952968L
>>> a = a +[3]
>>> a
[1, 2, 3]
>>> id(a)
43001480L

```

I know the we using `a.append(1)` or `a += [2]`,the `id(a)` is't changing.when using `a = a +[3]` the `id(a)` is changed.

Now , I using like this:

```
>>> c = ([], [])
>>> id(c)
43002440L
>>> c[0].append(1)
>>> c
([1], [])
>>> id(c)
43002440L
>>> c[0] += [2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> c
([1, 2], [])
>>> id(c)
43002440L

```

I using `c[0].append(1)` is right, when I trying `c[0] += [2]`, I got a type error.But I found the `c` value is changed.