Updating lists inside tuples

Question

If I have a list inside a tuple, and then attempt to concatenate that list with another, I get an exception:

>>> a = ([1], [2])
>>> a[1] += [3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

This is expected: the concatenation is creating a new list, and tuples are immutable. However, despite the exception, the tuple is updated:

>>> print(a)
([1], [2, 3])

Why is this the case?

This is yet another case where using `.append` or `.extend` is preferable to using `+=`. — PM 2Ring, Feb 12 '18 at 12:37

score 5 · Answer 1 · answered Feb 12 '18 at 12:31

5

That's because the concatenation (+) happens first, and then the assignment.

As the elements of the tuple here are lists which are mutable, the a[1] + [3] would succeed, but as the tuple itself is immutable, the assignment of the output of the concatenation to a[1] would fail (expectedly).

answered Feb 12 '18 at 12:31

heemayl

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details [tuples-and-sequences](https://docs.python.org/3.3/tutorial/datastructures.html#tuples-and-sequences) – Brown Bear Feb 12 '18 at 12:34
https://inventwithpython.com/blog/2018/02/05/python-tuples-are-immutable-except-when-theyre-mutable/ – Veera Balla Deva Feb 12 '18 at 12:36

Updating lists inside tuples

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