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3.5: Lists and Tuples

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  • zip is a built-in function that takes two or more sequences and “zips” them into a list of tuples where each tuple contains one element from each sequence. In Python 3, zip returns an iterator of tuples, but for most purposes, an iterator behaves like a list.

    This example zips a string and a list:

    >>> s = 'abc'
    >>> t = [0, 1, 2]
    >>> zip(s, t)
    [('a', 0), ('b', 1), ('c', 2)]

    The result is a list of tuples where each tuple contains a character from the string and the corresponding element from the list.

    If the sequences are not the same length, the result has the length of the shorter one.

    >>> zip('Anne', 'Elk')
    [('A', 'E'), ('n', 'l'), ('n', 'k')]

    You can use tuple assignment in a for loop to traverse a list of tuples:

    t = [('a', 0), ('b', 1), ('c', 2)]
    for letter, number in t:
        print number, letter

    Each time through the loop, Python selects the next tuple in the list and assigns the elements to letter and number. The output of this loop is:

    0 a
    1 b
    2 c

    If you combine zip, for and tuple assignment, you get a useful idiom for traversing two (or more) sequences at the same time. For example, has_match takes two sequences, t1 and t2, and returns True if there is an index i such that t1[i] == t2[i]:

    def has_match(t1, t2):
        for x, y in zip(t1, t2):
            if x == y:
                return True
        return False

    If you need to traverse the elements of a sequence and their indices, you can use the built-in function enumerate:

    for index, element in enumerate('abc'):
        print index, element

    The output of this loop is:

    0 a
    1 b
    2 c


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