Python Dictionary and Dictionary Methods: A Guide

In Python, dictionaries are ordered data structures that map keys to values. After defining a dictionary, it’s possible to add or update key-value pairs.

This Python dictionary tutorial covers:

• How to define a dictionary.
• How to access values and check for keys in a dictionary.
• How to add, update and delete keys from a dictionary.
• Dictionary methods.
• How to iterate through a dictionary.

With that, let’s get started.

How to Define a Dictionary in Python

Dictionaries are written within curly brackets {}.

# Define a dictionary code 
webstersDict = {'person': 'a human being',
                'marathon': 'a running race that is about 26 miles',
                'resist': 'to remain strong against the force',
                'run': 'to move with haste; act quickly'}

While the dictionary webstersDict used strings as keys in the dictionary, dictionary keys can be any immutable data type (numbers, strings, tuples, etc). Dictionary values can also be just about anything (integers, lists, functions, strings, etc).

For example, the dictionary below, genderDict has ints as keys and strings as values.

# Define a dictionary
genderDict = {0: 'male',
              1: 'female'}

It’s important to know that if you try to make a key a mutable data type (like a list), you will get an error.

# Failure to define a dictionary
webstersDict = {(1, 2.0): 'tuples can be keys',
                1: 'ints can be keys',
                'run': 'strings can be keys', 
                ['sock', 1, 2.0]: 'lists can NOT be keys'}

More on Python: 10 Ways to Convert Lists to Dictionaries in Python

How to Access Values and Check for Keys in a Python Dictionary

Access Values in a Dictionary

To access a dictionary value, you can use square brackets [].

For example, the code below uses the key ‘marathon’ to access the value ‘a running race that is about 26 miles’.

# Get value of the 'marathon' key
webstersDict['marathon']

Keep in mind that you will get a KeyError if you try to access a value for a key that does not exist. 

# Try to get value for key that does not exist
webstersDict['nonexistentKey']

In the dictionary methods section, you will see the utility of using the dictionary method get() to avoid KeyErrors.

Check If a Key Exists in a Dictionary

You can check if a key exists in a dictionary before trying to access it by using the in operator. The operator will return True if the key exists and False if it does not. For example, say we want to check if the key ‘marathon’ exists in the webstersDict dictionary, we would use the code:

print('marathon' in webstersDict)

#Output: True

How to Add, Update and Delete Keys from a Python Dictionary

Add or Update a Key

You can add a new key-value pair.

# add one new key value pair to a dictionary
webstersDict['shoe'] = 'an external covering for the human foot'

You can also update a key-value pair.

In the dictionary methods section, you will see that you can also add or update multiple key value pairs at a time using the dictionary update method.

Delete Keys from a Dictionary

It is possible to remove a key and its corresponding value from a dictionary using del.

# Remove the key 'resist' from the dictionary
del webstersDict['resist']

In the dictionary methods section, you will see that you can also delete keys using the dictionary pop method.

Python Dictionary Methods

Python dictionaries have different methods that help you modify a dictionary. This section of the tutorial just goes over various python dictionary methods.

Update Method

The update method is very useful for updating multiple key values pairs at a time. It takes a dictionary as an argument.

# Using update method to add two key value pairs at once
webstersDict.update({'ran': 'past tense of run',
                     'shoes': 'plural of shoe'})

Get Method

# Define a dictionary
storyCount = {'is': 100,
              'the': 90,
              'Michael': 12,
              'runs': 5}

The get method returns a value for a given key. If a key doesn’t exist, the dictionary will by default return None.

# Since the key 'Michael' exists, it will return the value 12
storyCount.get('Michael')

This method is very useful for looking up keys that you don’t know are in the dictionary to avoid KeyErrors.

You can also specify a default value to return if the key doesn’t exist.

# Make default value for key that doesn't exist 0.
storyCount.get('chicken', 0)

You can see the usefulness of this method if you try a Python Word Count.

Pop Method

The pop method removes a key and returns the value.

storyCount.pop('the')

Keys Method

The keys method returns the keys of the dictionary.

storyCount.keys()

Values Method

The values method returns the values in the dictionary.

storyCount.values()

Items Method

The items method returns a list-like object of tuples in which each tuple is of the form (key, value).

webstersDict.items()

More on Python: Python Lists and List Manipulation Explained

How to Iterate Through a Python Dictionary

You can iterate through the keys of a dictionary by using a for loop.

for key in storyCount:
   print(key)

You also iterate through the keys of a dictionary by using the keys method.

for key in storyCount.keys():
   print(key)

The for loop below uses the items method to access one (key, value) pair on each iteration of the loop.

for key, value in webstersDict.items():
    print(key, value)

Frequently Asked Questions