A set is an unordered collection of zero or more immutable Python data
objects. Sets do not allow duplicates and are written as commadelimited
values enclosed in curly braces. The empty set is represented by
set(). Sets are heterogeneous, and the collection can be assigned to
a variable as below.

>>> {3,6,"cat",4.5,False}
{False, 4.5, 3, 6, 'cat'}
>>> mySet = {3,6,"cat",4.5,False}
>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>>

Even though sets are not considered to be sequential, they do support a
few of the familiar operations presented earlier.
Table reviews
these operations and the following session gives examples of their use.
Operation Name 
Operator 
Explanation 
membership 
in 
Set membership 
length 
len 
Returns the cardinality of the set 
 
aset  otherset 
Returns a new set with all elements from both sets 
& 
aset & otherset 
Returns a new set with only those elements common to both sets 
 
aset  otherset 
Returns a new set with all items from the first set not in second 
<= 
aset <= otherset 
Asks whether all elements of the first set are in the second 
Operations on a Set in Python

>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>> len(mySet)
5
>>> False in mySet
True
>>> "dog" in mySet
False
>>>

Sets support a number of methods that should be familiar to those who
have worked with them in a mathematics setting.
Table
provides a summary. Examples of their use follow. Note that
union,
intersection,
issubset, and
difference all have operators
that can be used as well.
Method Name 
Use 
Explanation 
union 
aset.union(otherset) 
Returns a new set with all elements from both sets 
intersection 
aset.intersection(otherset) 
Returns a new set with only those elements common to both sets 
difference 
aset.difference(otherset) 
Returns a new set with all items from first set not in second 
issubset 
aset.issubset(otherset) 
Asks whether all elements of one set are in the other 
add 
aset.add(item) 
Adds item to the set 
remove 
aset.remove(item) 
Removes item from the set 
pop 
aset.pop() 
Removes an arbitrary element from the set 
clear 
aset.clear() 
Removes all elements from the set 

aset.clear() 
Removes all elements from the set 
Methods Provided by Sets in Python
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>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>> yourSet = {99,3,100}
>>> mySet.union(yourSet)
{False, 4.5, 3, 100, 6, 'cat', 99}
>>> mySet  yourSet
{False, 4.5, 3, 100, 6, 'cat', 99}
>>> mySet.intersection(yourSet)
{3}
>>> mySet & yourSet
{3}
>>> mySet.difference(yourSet)
{False, 4.5, 6, 'cat'}
>>> mySet  yourSet
{False, 4.5, 6, 'cat'}
>>> {3,100}.issubset(yourSet)
True
>>> {3,100}<=yourSet
True
>>> mySet.add("house")
>>> mySet
{False, 4.5, 3, 6, 'house', 'cat'}
>>> mySet.remove(4.5)
>>> mySet
{False, 3, 6, 'house', 'cat'}
>>> mySet.pop()
False
>>> mySet
{3, 6, 'house', 'cat'}
>>> mySet.clear()
>>> mySet
set()
>>>

From:http://interactivepython.org/courselib/static/pythonds/Introduction/introduction.html
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