# Data Structures — 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. Curly bra

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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.

Curly braces or the set function can be used to create sets. Note: to create an empty set you have to use set(), not {}; the latter creates an empty dictionary, a data structure that we discuss in the next section.

Because sets are unordered, iterating over them or printing them can produce the elements in a different order than you expect.

Here is a brief demonstration

&gt;&gt;&gt; basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'} &gt;&gt;&gt; print(basket) # show that duplicates have been removed {'orange', 'banana', 'pear', 'apple'} &gt;&gt;&gt; 'orange' in basket # fast membership testing True &gt;&gt;&gt; 'crabgrass' in basket False

&gt;&gt;&gt; # Demonstrate set operations on unique letters from two words &gt;&gt;&gt; &gt;&gt;&gt; a = set('abracadabra') &gt;&gt;&gt; b = set('alacazam') &gt;&gt;&gt; a # unique letters in a {'a', 'r', 'b', 'c', 'd'} &gt;&gt;&gt; a - b # letters in a but not in b {'r', 'd', 'b'} &gt;&gt;&gt; a | b # letters in a or b or both {'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'} &gt;&gt;&gt; a &amp; b # letters in both a and b {'a', 'c'} &gt;&gt;&gt; a ^ b # letters in a or b but not both {'r', 'd', 'b', 'm', 'z', 'l'}

Similarly to list comprehensions , set comprehensions are also supported, including comprehensions with unpacking

&gt;&gt;&gt; a = {x for x in 'abracadabra' if x not in 'abc'} &gt;&gt;&gt; a {'r', 'd'}

&gt;&gt;&gt; fruits = [{'apple', 'avocado', 'apricot'}, {'banana', 'blueberry'}] &gt;&gt;&gt; {fruit for fruit in fruits} {'blueberry', 'banana', 'avocado', 'apple', 'apricot'}

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