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5.4 Sets

Python also includes a data type for sets. A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference.

Here is a brief demonstration:

    >>> basket = ['apple', 'orange', 'apple', 'pear',
                  'orange', 'banana']
    >>> fruit = set(basket)  # make set without duplicates
    >>> fruit
    set(['orange', 'pear', 'apple', 'banana'])
    >>> 'orange' in fruit    # fast membership testing
    True
    >>> 'crabgrass' in fruit
    False
    
    >>> # Demonstrate set operations on unique letters
        # from two words
    ...
    >>> a = set('abracadabra')
    >>> b = set('alacazam')
    >>> a      # unique letters in a
    set(['a', 'r', 'b', 'c', 'd'])
    >>> a - b  # letters in a but not in b
    set(['r', 'd', 'b'])
    >>> a | b  # letters in either a or b
    set(['a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'])
    >>> a & b  # letters in both a and b
    set(['a', 'c'])
    >>> a ^ b  # letters in a or b but not both
    set(['r', 'd', 'b', 'm', 'z', 'l'])