Dictionaries

5.7. Dictionaries#

A dictionary (mostly unrelated to a data dictionary which we saw earlier) is a versatile data structure that allows you to store data as key-value pairs. It is similar to a real-world dictionary where you look up a word (the key) to find its definition (the value).

Dictionaries are:

Structured with key-value pairs. Each dictionary consists of a unique key and its associated values. Keys must be unique.
Unordered. There is no innate order for the key-value pairs in a dictionary. In recent versions of Python dictionaries will maintain the order which the key-value pairs were inserted, but this is not true in other languages.
Mutable. Unlike records, you can add, remove or modify entries after the dictionary is created.
Efficiently accessed. Dictionaries are typically optimised for the fast retrieval based on their keys. In Python this is achieved by implementing them as a hash map, which is a data structure you’ll learn about later.

5.7.1. Creating A Dictionary#

To create a dictionary in python you can use braces (i.e. {}). Elements in the dictionary are key value pairs where the key and pair are separated by a colon : and elements are separated by a comma ,. The format of a dictionary is as follows

dictionary = {key_1:value_1,
                ...
            key_n:value_n}

Here’s an example

wattle_high_school = {
    "Name": "Wattle High School",
    "Students": 84,
    "Motto": "Aliquam libero sit amet!",
}

We can create an empty dictionary just using braces by themselves, i.e. {} the same way we can make an empty list with [] and an empty tuple with ().

5.7.2. Accessing And Modifying A Dictionary#

Accessing values from a dictionary is easy! Simply provide a key as an index:

wattle_high_school = {'Name': 'Wattle High School',
                      'Students': 84,
                      'Motto': 'Aliquam libero sit amet!'}

print(wattle_high_school['Students'])

To add or modify a key-value pair we access the element we want and overwrite it.

dictionary[key] = value

wattle_high_school = {'Name': 'Wattle High School',
                      'Students': 84,
                      'Motto': 'Aliquam libero sit amet!'}

# Add a key-value pair
wattle_high_school['Colours'] = 'Green and yellow'
print(wattle_high_school['Colours'])

# Modify a key-value pair
wattle_high_school['Students'] = 88
print(wattle_high_school['Students'])

Green and yellow
88

To delete a key-value pair we use del and access the element we want to delete.

del dictionary[key]

wattle_high_school = {'Name': 'Wattle High School',
                      'Students': 84,
                      'Motto': 'Aliquam libero sit amet!'}

# Remove a key-value pair
del wattle_high_school['Motto']

print(wattle_high_school)

{'Name': 'Wattle High School', 'Students': 84}

Dictionaries are typically used in cases where you want to represent structured data (like a record) but you want mutability or in cases in which you want to track relationships between quantities (e.g. your contacts list).

There are also a few functions which allow you to iterate through the dictionary.

.keys() will give you a list of keys (in a random order)
.values() will give you a list of values (in a random order)
.items() will give you a list of key-value tuples (in a random order)

wattle_high_school = {'Name': 'Wattle High School',
                      'Students': 84,
                      'Motto': 'Aliquam libero sit amet!'}
print(wattle_high_school.keys())
print(wattle_high_school.values())
print(wattle_high_school.items())

dict_keys(['Name', 'Students', 'Motto'])
dict_values(['Wattle High School', 84, 'Aliquam libero sit amet!'])
dict_items([('Name', 'Wattle High School'), ('Students', 84), ('Motto', 'Aliquam libero sit amet!')])

You can use any of these to iterate through the dictionary.

With .keys()

wattle_high_school = {'Name': 'Wattle High School',
                      'Students': 84,
                      'Motto': 'Aliquam libero sit amet!'}
for key in wattle_high_school.keys(): # for key in wattle_high_school also works
    print(key)

Name
Students
Motto

With .values()

wattle_high_school = {'Name': 'Wattle High School',
                      'Students': 84,
                      'Motto': 'Aliquam libero sit amet!'}
for value in wattle_high_school.values():
    print(value)

Wattle High School
84
Aliquam libero sit amet!

With .items()

wattle_high_school = {'Name': 'Wattle High School',
                     'Students': 84,
                     'Motto': 'Aliquam libero sit amet!'}
for key, value in wattle_high_school.items():
    print('{}: {}'.format(key, value))

Name: Wattle High School
Students: 84
Motto: Aliquam libero sit amet!

5.7.3. Dictionary Errors#

If you try to access a dictionary element using a key that does not exist, you will get a KeyError.

wattle_high_school = {
    "Name": "Wattle High School",
    "Students": 84,
    "Motto": "Aliquam libero sit amet!",
}
print(wattle_high_school["Address"])

Traceback (most recent call last):
    File "/home/main.py", line 4, in <module>
        print(wattle_high_school['Address'])
            ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
    KeyError: 'Address'