Question

I just came across something that was quite strange.

>>> t = ([],)
>>> t[0].append('hello')
>>> t
(['hello'],)
>>> t[0] += ['world']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> t
(['hello', 'world'],)

Why does it raise TypeError and yet change the list inside the tuple?

Answer 1

As I started mentioning in comment, += actually modifies the list in-place and then tries to assign the result to the first position in the tuple. From the data model documentation:

These methods are called to implement the augmented arithmetic assignments (+=, -=, =, /=, //=, %=, *=, <<=, >>=, &=, ^=, |=). These methods should attempt to do the operation in-place (modifying self) and return the result (which could be, but does not have to be, self).

+= is therefore equivalent to:

t[0].extend(['world']);
t[0] = t[0];

So modifying the list in-place is not problem (1. step), since lists are mutable, but assigning the result back to the tuple is not valid (2. step), and that’s where the error is thrown.