Python Dictionary and Dictionary Methods: A Guide

This Python dictionary tutorial covers how to define and access values in a dictionary, how to iterate through a dictionary and various dictionary methods.

Written by Michael Galarnyk
Published on Jan. 27, 2023
Python Dictionary and Dictionary Methods: A Guide
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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.

What Is a Python Dictionary?

Python dictionaries are ordered data structures that map keys to values. Dictionary keys can be any immutable data type, such as numbers, strings, tuples, etc, while dictionary values can be just about anything from integers to lists, functions, strings, etc. Dictionaries are written within curly brackets {}. 

This Python dictionary tutorial covers:

  • How to define a dictionary.
  • How to access values 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.

 

Define a Dictionary

Dictionaries are written within curly brackets {}.

Define a dictionary. Keys are in red. Values are in blue.
Define a dictionary. Keys are in red. Values are in blue. | Image: Michael Galarnyk
# 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'}
Failure to define a dictionary with a list as a key. Lists are not immutable. | Image: Michael Galarnyk
Failure to define a dictionary with a list as a key. Lists are not immutable. | Image: Michael Galarnyk

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

 

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']
Access the key ‘marathon.’
Access the key ‘marathon.’ | Image: Michael Galarnyk

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']
KeyError will result if you try and look up a key that does not exist.
KeyError will result if you try and look up a key that does not exist. | Image: Michael Galarnyk

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

 

Add, Update, and Delete Keys from a 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'
Add the new key ‘shoe’ to the dictionary. The new key ‘shoe’ is enclosed in the red rectangle.
Add the new key ‘shoe’ to the dictionary. The new key ‘shoe’ is enclosed in the red rectangle. | Image: Michael Galarnyk

You can also update a key-value pair.

Update the dictionary key ‘marathon.’
Update the dictionary key ‘marathon.’ | Image: Michael Galarnyk

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 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']
Remove the key ‘resist’ from the dictionary webstersDict.
Remove the key ‘resist’ from the dictionary webstersDict. | Image: Michael Galarnyk

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

 

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'})
Added the keys ‘ran’ and ‘shoes’ to the dictionary. | Image: Michael Galarnyk
Added the keys ‘ran’ and ‘shoes’ to the dictionary. | Image: Michael Galarnyk

 

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')
Since the key ‘Michael’ exists, it returns the value 12. If ‘Michael’ didn’t exist, it would return None.
Since the key ‘Michael’ exists, it returns the value 12. If ‘Michael’ didn’t exist, it would return None. | Image: Michael Galarnyk

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

They key ‘chicken’ does not exist.
They key ‘chicken’ does not exist. | Image: Michael Galarnyk

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)
Using the get method to see if ‘chicken’ is in the dictionary.
Using the get method to see if ‘chicken’ is in the dictionary. | Image: Michael Galarnyk

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')
Dictionary before and after removing the key ’the’ from the dictionary.
Dictionary before and after removing the key ’the’ from the dictionary. | Image: Michael Galarnyk

 

Keys Method

The keys method returns the keys of the dictionary.

storyCount.keys()
Using the keys method.
Using the keys method. | Image: Michael Galarnyk

 

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()
Value method example.
Value method example. | Image: Michael Galarnyk

More on Python: Python Lists and List Manipulation Explained

 

Iterate Through a Dictionary

A tutorial on Python dictionaries and dictionary manipulation methods. | Video: Michael Galarnyk

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

for key in storyCount:
   print(key)
Iterate through the keys of the dictionary.
Iterate through the keys of the dictionary. | Image: Michael Galarnyk

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

for key in storyCount.keys():
   print(key)
Iterate through the keys of the dictionary.
Iterate through the keys of the dictionary. | Image: Michael Galarnyk

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)
Iterate through the key, value pairs of a dictionary.
Iterate through the key, value pairs of a dictionary. | Image: Michael Galarnyk
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