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Working with Dictionaries in Python

Last Updated on Wednesday 5th Oct 2022

Dictionaries

A dictionary is a collection of unordered, modifiable(mutable) paired (key: value) data type.

Creating a Dictionary

To create a dictionary we use curly brackets, {}.

			
					# empty one
empty_dicto = {}

			
	

			
	
	


			
					dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3'}

Example:

			
					book = {
    'name':'The Heart Principle',
    'author':'Helen Hoang',
    'published': '31 August 2021',
    'genres': ['Romance novel', 'Fiction', 'Contemporary romance'],
}

The dictionary above shows that a value could be any different data type.

  • string
  • boolean
  • list
  • tuple
  • set or a dictionary.

Dictionary Length

It checks the number of key: value pairs in the dictionary.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(len(dicto)) # 4

Example:

			
					book = {
    'name':'The Heart Principle',
    'author':'Helen Hoang',
    'published': '31 August 2021',
    'genres': ['Romance novel', 'Fiction', 'Contemporary romance'],
}
print(len(book)) # 4

Accessing Dictionary Items

We can access Dictionary items by referring to its key name.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(dicto['key1']) # value1
print(dicto['key4']) # value4

Example:

			
					book = {
    'name':'The Heart Principle',
    'author':'Helen Hoang',
    'published': '31 August 2021',
    'genres': ['Romance novel', 'Fiction', 'Contemporary romance'],
}

print(book['name']) # The Heart Principle
print(book['author']) # Helen Hoang
print(book['genres'][1]) # Fiction
  • Accessing an item by key name raises an error if the key does not exist.
  • To avoid this use get method.It returns None, which is a NoneType object data type, if the key does not exist.
			
					book = {
    'name':'The Heart Principle',
    'author':'Helen Hoang',
    'published': '31 August 2021',
    'genres': ['Romance novel', 'Fiction', 'Contemporary romance'],
}

print(book.get('name')) # The Heart Principle
print(book.get('author')) # Helen Hoang
print(book.get('genres')) # ['Romance novel', 'Fiction', 'Contemporary romance']

Adding Items to a Dictionary

We can add new key and value pairs to a dictionary

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dicto['key5'] = 'value5'

Example:

			
					book = {
    'name':'The Heart Principle',
    'author':'Helen Hoang',
    'published': '31 August 2021',
    'genres': ['Romance novel', 'Fiction', 'Contemporary romance'],
}

book['genres'].append('something')

Modifying Items in a Dictionary

We can modify items in a dictionary

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dicto['key1'] = 'value-one'

Example:

			
					book = {
    'name': 'The Heart Principle',
    'author': 'Helen Hoang',
    'published': '31 August 2021',
    'genres': ['Romance novel', 'Fiction', 'Contemporary romance'],
}

book['name'] = 'Beautiful Country: A Memoir'

Checking Keys in a Dictionary

We use the in operator to check if a key exist in a dictionary.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print('key2' in dicto) # True
print('key5' in dicto) # False

Getting Dictionary Keys as a List

The keys() method gives us all the keys of a a dictionary as a list.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
keys = dicto.keys() # dict_keys(['key1', 'key2', 'key3', 'key4'])

Getting Dictionary Values as a List

The values method gives us all the values of a a dictionary as a list.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
values = dicto.values()
print(values)     # dict_values(['value1', 'value2', 'value3', 'value4'])

Deleting a Dictionary

If we do not use the dictionary we can delete it completely

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
del dicto

Removing Key and Value Pairs from a Dictionary

  • pop(key): removes the item with the specified key name:
  • popitem(): removes the last item
  • del: removes an item with specified key name
			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dicto.pop('key1') # removes key1 item
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dicto.popitem() # removes the last item
del dicto['key2'] # removes key2 item

Example:

			
					book = {
    'name': 'The Heart Principle',
    'author': 'Helen Hoang',
    'published': '31 August 2021',
    'genres': ['Romance novel', 'Fiction', 'Contemporary romance'],
}

book.pop('name')        # Removes the name of the book
del book['published']        # Removes the published date of the book

Changing Dictionary to a List of Items

The items() method changes dictionary to a list of tuples.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(dicto.items()) # dict_items([('key1', 'value1'), ('key2', 'value2'), ('key3', 'value3'), ('key4', 'value4')])

Clearing a Dictionary

If we don't want the items in a dictionary we can clear them using clear() method.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(dicto.clear()) # None

Copy a Dictionary

We can copy a dictionary using a copy() method.Using copy we can avoid mutation of the original dictionary.

			
					# syntax
dicto = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dicto_copy = dicto.copy() # {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}