In [99]:

print S

set(['a', 'b', 12, 'hello', (1, 2, 3)])

In [100]:

The cardinality of a set can be checked using 'len':

  File "<ipython-input-100-5b525216f1fb>", line 1
    The cardinality of a set can be checked using 'len':
                  ^
SyntaxError: invalid syntax

In [101]:

print len(S)

Since a list is mutable, it cannot be a member of a cell:

In [102]:

notS={[1,2,3]}

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-102-fa57760b3459> in <module>()
----> 1 notS={[1,2,3]}

TypeError: unhashable type: 'list'

Whereas a tuple can be:

In [103]:

okS={tuple([1,2,3])}

In [104]:

print okS

set([(1, 2, 3)])

Sets don't contain duplicate members:

In [105]:

anotherS={'a','b','a'}

In [106]:

print anotherS

set(['a', 'b'])

Question: Can Python sets contain other Python sets?¶

Question: Why can't Python sets contain mutable objects?¶

Hint: Sets do not allow repeats!

In [107]:

S=set(tuple(range(10)))

In [108]:

print S

set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

In [109]:

print 0 in S

True

In [110]:

print -1 not in S

True

In [111]:

S.isdisjoint({13,14})

Out[111]:

True

In [112]:

S.issubset({1,2,3}) #Check if S is a subset of {1,2,3}

Out[112]:

False

In [113]:

S.issubset(set(tuple(range(100))))

Out[113]:

True

In [114]:

S<=set(tuple(range(100))) #Same as issubset above

Out[114]:

True

In [115]:

S<set(tuple(range(100))) #Proper subset test i.e., set <= other and set != other

Out[115]:

True

In [116]:

T=S.union({-1,-2}) #Equivalent operator: set | other I ...

In [117]:

print T

set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -2, -1])

In [118]:

T=S.intersection({-1,0,1}) #Equivalent operator: set & other & ...

In [119]:

print T

set([0, 1])

In [120]:

T=S.difference([-1,0,1,2]) #Equivalent operator: set - other - ...

In [121]:

print T

set([3, 4, 5, 6, 7, 8, 9])

In [122]:

T=S.symmetric_difference([-1,0,1,2]) #Equivalent operator: set ^ other. Returns elements that are in either set but not both.

In [123]:

print T

set([3, 4, 5, 6, 7, 8, 9, -1])

Note that in the above two examples, we passed a list. All the above functions accept any 'iterable' as an argument but their operator counter parts will operate only on sets to avoid unexpected results from operations such as set('abc') & 'cbs'. For more details see Python documentation on sets.

There are multiple ways of updating a set:

In [124]:

S.update([-2,-1]) #Same as union. You can also use S|={-2,-1}.

In [125]:

print S

set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1, -2])

Other operators can also be used similarly.

Elements can be added or removed to a set using 'add' or 'remove'.

In [126]:

S.add(100)

In [127]:

print S

set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1, 100, -2])

In [128]:

S.remove(-1)

In [129]:

print S

set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 100, -2])

'remove(elem)' raise a KeyError exception if the element is not contained in the set.

In [130]:

S.remove(1000)

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-130-7ff7c8e41a26> in <module>()
----> 1 S.remove(1000)

KeyError: 1000

'discard' does the same as 'remove' without raising any exception in case the element is not found.

In [131]:

S.discard(1000)

'pop' removes and returns an arbitrary element from the set and raises KeyError if the set is empty.

In [132]:

while(1):
    print S
    print S.pop()

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-132-efe053d450e2> in <module>()
      1 while(1):
      2     print S
----> 3     print S.pop()

KeyError: 'pop from an empty set'

set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 100, -2])
0
set([1, 2, 3, 4, 5, 6, 7, 8, 9, 100, -2])
1
set([2, 3, 4, 5, 6, 7, 8, 9, 100, -2])
2
set([3, 4, 5, 6, 7, 8, 9, 100, -2])
3
set([4, 5, 6, 7, 8, 9, 100, -2])
4
set([5, 6, 7, 8, 9, 100, -2])
5
set([6, 7, 8, 9, 100, -2])
6
set([7, 8, 9, 100, -2])
7
set([8, 9, 100, -2])
8
set([9, 100, -2])
9
set([100, -2])
100
set([-2])
-2
set([])

'clear' removes all elements from the set. Note that S={} will not make S empty because {} is a dictionary. If you want to create an empty set use:

Sets in Python¶

Question: Can Python sets contain other Python sets?¶

Question: Why can't Python sets contain mutable objects?¶

Set operations:¶