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Guide to Python Dictionary and Dictionary Methods

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.

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 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.

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

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.

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

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.

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

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'})

# 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.

The pop method removes a key and returns the value.

storyCount.pop('the')

The keys method returns the keys of the dictionary.

storyCount.keys()

The values method returns the values in the dictionary.

storyCount.values()

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

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)

A Python dictionary is defined using curly brackets ({}) with key-value pairs. For example:

webstersDict = {'person': 'a human being', 'marathon': 'a running race'}

No, Python dictionary keys must be immutable data types (e.g., numbers, strings, tuples). Mutable data types like lists cannot be dictionary keys.

To access a value in a Python dictionary, use square brackets ([]) with the associated key. For example:

webstersDict['marathon']  # Returns 'a running race'

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