3.4. Tuples

Lists allow storage of items that can change over time. However, sometimes the developer requires a list that remains constant over time. Python defines tuples to accommodate this need, and this is also the reason why tuples are faster than lists. Objects that remain constant over time are referred to as immutable in Python. Thus, a tuple can be defined as an immutable list. In terms of code, a tuple looks just like a list except the square brackets are replaced by parentheses. For example:

>>> a = (100,20,'a')
>>> print a[0]
100
>>> print a[2]
'a'
>>> a[2] = 9
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

As shown above, the individual elements can be accessed by specifying the element index within square brackets, exactly as lists. Note how Python reports an error whenever an attempt is made to alter the value of an element. While the elements within a tuple cannot be changed, the variable holding the tuple can always be assigned a new tuple, as shown below:

>>> a = (100,20,'a')
>>> a = (0,1,2,3,4)
>>> a
(0, 1, 2, 3, 4)