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3.1: Tuples Are Immutable

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  • A tuple is a sequence of values. The values can be any type, and they are indexed by integers, so in that respect tuples are a lot like lists. The important difference is that tuples are immutable.

    Syntactically, a tuple is a comma-separated list of values:

    >>> t = 'a', 'b', 'c', 'd', 'e'

    Although it is not necessary, it is common to enclose tuples in parentheses:

    >>> t = ('a', 'b', 'c', 'd', 'e')

    To create a tuple with a single element, you have to include a final comma:

    >>> t1 = 'a',
    >>> type(t1)
    <type 'tuple'>

    A value in parentheses is not a tuple:

    >>> t2 = ('a')
    >>> type(t2)
    <type 'str'>

    Another way to create a tuple is the built-in function tuple. With no argument, it creates an empty tuple:

    >>> t = tuple()
    >>> print t

    If the argument is a sequence (string, list or tuple), the result is a tuple with the elements of the sequence:

    >>> t = tuple('lupins')
    >>> print t
    ('l', 'u', 'p', 'i', 'n', 's')

    Because tuple is the name of a built-in function, you should avoid using it as a variable name.

    Most list operators also work on tuples. The bracket operator indexes an element:

    >>> t = ('a', 'b', 'c', 'd', 'e')
    >>> print t[0]

    And the slice operator selects a range of elements.

    >>> print t[1:3]
    ('b', 'c')

    But if you try to modify one of the elements of the tuple, you get an error:

    >>> t[0] = 'A'
    TypeError: object doesn't support item assignment

    You can’t modify the elements of a tuple, but you can replace one tuple with another:

    >>> t = ('A',) + t[1:]
    >>> print t
    ('A', 'b', 'c', 'd', 'e')
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