List mutation inside a tuple in Python

Question

I am trying to understand the following error in python:

In []: tup = tuple(['foo', [1,2], True])
In []: tup[1]+=[2]

This results in the following error:

TypeError: 'tuple' object does not support item assignment

I understand this because tuples are not mutable. However, the second element of the tuple is now mutated as desired.

In []: tup
Out[]: ('foo', [1, 2, 2], True)

Can someone explain what happened here. I got an error, but the code did exactly what I wanted. What failed to execute the code that displays the error?

Also, I understand that the append method can do the same.

In []: tup[1].append(2)
In []: tup
Out[]: ('foo', [1, 2, 2, 2], True)

But I still want to understand what happened in the first case above.