Overview

Teaching: 10 min
Exercises: 5 min

Questions

• What is a set?

• What is a dictionary?

Objectives

• Explain the features of sets and dictionaries.

• Understand how dictionaries map keys to values

• Learn how to add and get items from sets and dictionaries.

Sets

Sets are unordered collections of unique items

• Construct a set with braces: my_set = {object1, object2, object3, ...}
  • You can also use the built-in set() function to make a set from another iterable: my_set = set("abc")
• Sets in Python behave like mathematical sets
  • Support common set operations (as methods):
  • my_set.union(), my_set.difference(), my_set.intersect()

set1 = set("abc")
set2 = set("acb")
set3 = set("aabbbcc")
print(set1, set2, set3)

{'b', 'c', 'a'} {'b', 'c', 'a'} {'b', 'c', 'a'}

• Add items to a set using the add() and update() methods

my_set = {'a', 'b', 'c'}
my_set.add('d')
print(my_set)

my_set.update({'e', 'f', 'g'})
print(my_set)

{'a', 'c', 'd', 'b'}
{'f', 'g', 'b', 'e', 'c', 'd', 'a'}

• Get an element from a set using pop()
• This returns an element from the set and removes it

some_set = set(['UGA', 'UAG', 'UAA'])
print(some_set)
print(some_set.pop())
print(some_set)

{'UAG', 'UAA', 'UGA'}
UAG
{'UAA', 'UGA'}

What objects can go in a set?

Surprisingly, not every type of object can be added to a set

my_set = {[1, 2]}

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-bf858b2308e0> in <module>()
----> 1 my_set = {[1, 2]}
 
TypeError: unhashable type: 'list'

• The error means that python cannot add a list to a set
• Only objects that cannot change can be added to sets

Dictionaries

Dictionaries add meaningful labels

• Once you start binding data together, it is often useful to add semantic meaning
• Dictionaries allow you to label the entries that are bound together
  • Be sure to choose meaningful names!
• Dictionaries are mutable, you can change them
  • Because of meaningful labels, this is less dangerous
• Dictionaries map keys to values: my_dict = {key1:value1, key2:value2, ...}
  • As with sets, dictionary keys must be guaranteed not to change

name = {'first': 'Paul', 'last': 'Wilson'}
print(name['first'], name['last'])
name['last'] = 'Nagus-Wilson'
print(name['first'], name['last'])

Paul Wilson
Paul Nagus-Wilson

Entries can be added to a dictionary

• As long as the variable is already of a dictionary type, new entries can be added as if they were already there
• Only one item can exist for each unique key
  • Assigning a new value to an existing key overwrites the old value

my_name = {'first': 'Paul', 'last': 'Wilson'}
my_name['middle'] = 'Philip'
print(my_name)
my_name['last'] = 'Nagus-Wilson'
print(my_name)
your_name['first'] = 'Henry'

{'first': 'Paul', 'last': 'Wilson', 'middle': 'Philip'}
{'first': 'Paul', 'last': 'Nagus-Wilson', 'middle': 'Philip'}

--------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-97-cc04cabb3603> in <module>()
      1 my_name = {'first': 'Paul', 'last': 'Wilson'}
      2 my_name['middle'] = 'Philip'
----> 3 your_name['first'] = 'Henry'

NameError: name 'your_name' is not defined

Use del to remove items from a dictionary entirely.

my_name = {'first': 'Paul', 'middle': 'Philip', 'last': 'Wilson'}
print(my_name)
del my_name['middle']
print(my_name)

{'first': 'Paul', 'middle': 'Philip', 'last': 'Wilson'}
{'first': 'Paul', 'last': 'Wilson'}

Various ways to access all the entries in a dictionary

• The names of each entry are called keys
• The values of each entry are called values

name = {'first': 'Paul', 'last': 'Wilson'}
print(name.keys())
print(name.values())

dict_keys(['first', 'last'])
dict_values(['Paul', 'Wilson'])

Key Points

• A set is an unordered, unique collection of immutable objects.

• A dictionary is a mapping between objects, from keys to values.

• Dictionary keys must be immutable.

• Get a value from a dictionary with my_dict[some_key]. Add an item with my_dict[some_key] = some_value.

• Dictionaries and sets are mutable.