Python Language Tutorial => Tuples are immutable

Example

One of the main differences between lists and tuples in Python is that tuples are immutable, that is, one cannot add or modify items once the tuple is initialized. For example:

>>> t = (1, 4, 9)
>>> t[0] = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

Similarly, tuples don't have .append and .extend methods as list does. Using += is possible, but it changes the binding of the variable, and not the tuple itself:

>>> t = (1, 2)
>>> q = t
>>> t += (3, 4)
>>> t
(1, 2, 3, 4)
>>> q
(1, 2)

Be careful when placing mutable objects, such as lists, inside tuples. This may lead to very confusing outcomes when changing them. For example:

>>> t = (1, 2, 3, [1, 2, 3])
(1, 2, 3, [1, 2, 3])
>>> t[3] += [4, 5]

Will both raise an error and change the contents of the list within the tuple:

TypeError: 'tuple' object does not support item assignment
>>> t
(1, 2, 3, [1, 2, 3, 4, 5])

You can use the += operator to "append" to a tuple - this works by creating a new tuple with the new element you "appended" and assign it to its current variable; the old tuple is not changed, but replaced!

This avoids converting to and from a list, but this is slow and is a bad practice, especially if you're going to append multiple times.

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Python Language Tuple Tuples are immutable

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