Set theory in mathematics deals with groups of objects. It describes the relationship of an object with a set or group. Python also implements the set as standard python module. The sets module provides classes for constructing and manipulating unordered collections of unique elements.

Common uses include membership testing, removing duplicates from a sequence, and computing standard math operations on sets such as intersection, union, difference, and symmetric difference.

Non operator versions like union(), intersection(), difference(), symmetric_difference(), issubset(), issuperset() can take any iterable as argument. So it is recommended to use the non operator versions to be explicit and better readability.

Creating a set

set can be created using {} or by using set(), while working with set always use ‘set’ to be explicit and better readability.

developers = {'Alex', 'Dinesh', 'Rajesh', 'Maya', 'Suresh', 'Yuva'}

developers_list = ['Alex', 'Dinesh', 'Rajesh', 'Maya', 'Suresh', 'Yuva']
developers = set(developers_list) # set method takes an iterable

php_developers = set(['Dinesh', 'Rajesh', 'Maya', 'Suresh'])
python_developers = set(['Maya', 'Siva', 'Suresh', 'Yuva'])

Now, we have two sets php developers and python developers.

Basic operations

total_members = len(python_developers)
print "{} people knows Python".format(total_members)

for dev in python_developers:
    print dev

if 'Maya' in python_developers:
    print 'Maya is a member of python_developers'

if 'Shri' not in python_developers:
    print 'Magesh is not a member of python_developers'

Union operation using set

Union gives the developers who knows php or python or both.

# people who knows php or python or both

# using union() method
knows_php_or_python_or_both = php_developers.union(python_developers)
print knows_php_or_python_or_both

# using | operator
knows_php_or_python_or_both = php_developers | python_developers
print knows_php_or_python_or_both

Intersection operation using set

Intersection gives only the developers who knows both php and python.

# People who knows both php and python

# using intersection() method
both_php_and_python = php_developers.intersection(python_developers)
print both_php_and_python

# using & operator
both_php_and_python = php_developers & python_developers
print both_php_and_python

Difference operation using set

Difference of php developers and python developers gives only the developers who knows php.

# people who knows only php
# using difference() method
diff = php_developers.difference(python_developers)
print diff

# using - operator
diff = php_developers - python_developers
print diff

Symmetric Difference operation using set

Symmetric difference gives the developers who knows only php or python but not both.

# people who knows only php or python but not both

# using symmetric_difference() method
symmetric_diff = php_developers.symmetric_difference(python_developers)
print symmetric_diff

# using ^ operator
symmetric_diff = php_developers ^ python_developers
print symmetric_diff

Putting it all together

php_developers = set(['Dinesh', 'Rajesh', 'Maya', 'Suresh'])
python_developers = set(['Maya', 'Siva', 'Suresh', 'Yuva'])

total_members = len(python_developers)
print "{} people knows Python".format(total_members)

for dev in python_developers:
    print dev

if 'Maya' in python_developers:
    print 'Maya is a member of python_developers'

if 'Shri' not in python_developers:
    print 'Shri is not a member of python_developers'

# people who knows php or python or both

# using union() method
knows_php_or_python_or_both = php_developers.union(python_developers)
print knows_php_or_python_or_both

# using | operator
knows_php_or_python_or_both = php_developers | python_developers
print knows_php_or_python_or_both

# People who knows both php and python

# using intersection() method
both_php_and_python = php_developers.intersection(python_developers)
print both_php_and_python

# using & operator
both_php_and_python = php_developers & python_developers
print both_php_and_python

# using difference() method
diff = php_developers.difference(python_developers)
print diff

# using - operator
diff = php_developers - python_developers
print diff

# symmetric difference

# using symmetric_difference() method
symmetric_diff = php_developers.symmetric_difference(python_developers)
print symmetric_diff

# using ^ operator
symmetric_diff = php_developers ^ python_developers
print symmetric_diff

Tips and tricks

Find unique letters in a string

set('hello world') # set(['', 'e', 'd', 'h', 'l', 'o', 'r', 'w'])

Remove / find duplicates in a list/tuple

l = ['a', 'b', 'c', 'd', 'a', 'b']
set(l) # set(['a', 'c', 'b', 'd'])

Check if subset

issubset() checks if all items in the set exists in other set or an iterable.

# is subset
# s.issubset(t) test whether every element in s is in t

# using issubset() method
is_subset = python_developers.issubset(php_developers)
print is_subset

# using <= operator
is_subset = python_developers <= php_developers
print is_subset

Check if superset

issuperset() checks if all items in given iterable exists in the set.

# is superset
# s.issuperset(t) - test whether every element in t is in s
# using issuperset() method
is_superset = python_developers.issuperset(php_developers)
print is_superset

# using >= operator
is_superset = python_developers >= php_developers
print is_superset

Check if a list exists in another list

list1 = ['a', 'b']
set1 = set(list1)

list2 = ['a', 'b', 'c']
set1.issubset(list2)  # all items in list1 exists in list2