Python Dictionaries

Python dictionaries are a powerful data structure used to store key-value pairs. They are mutable, unordered, and can contain any number of key-value pairs. They are similar to lists, but unlike lists, dictionaries use keys to access the values, rather than using an index. The keys in a dictionary must be unique and immutable, while the values can be of any data type. Dictionaries are denoted by curly braces {} and use a colon : to separate the key and value.

1dict_literal =  {'name':  'William',  'age':  22}

Dictionaries are often used to store data in a structured way, such as a collection of user information or a database of products. The keys in a dictionary must be unique and immutable, while the values can be of any data type.

Creating a Dictionary

To create a dictionary, you can use the following syntax:

1# Empty dictionary
2my_dict = {}
3# Dictionary with initial values
4my_dict = {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}

You can also create a dictionary using the dict() constructor

1# Using dict() constructor
2my_dict = dict(key1='value1', key2='value2', key3='value3')

Python dictionary comprehension

In addition, you can use dictionary comprehension to create a new dictionary by applying an operation to each key-value pair of an existing dictionary.

1numbers =  {'a':  1,  'b':  2,  'c':  3}
2squared_numbers =  {k: v**2  for k, v in numbers.items()}
3print(squared_numbers)  # Output: {'a': 1, 'b': 4, 'c': 9}

Accessing Values

To access a value in a dictionary, you can use the key as the index:

1# Accessing values
2my_dict = {'name': 'William', 'age': 25, 'city': 'New York'}
3print(my_dict['name'])  # Output: William

Python also provides a method called get() which allows you to access the value of a key without raising an error if the key is not found in the dictionary. This can be useful when you're working with dictionaries that may contain keys that don't always exist.

You can also use the get() method to retrieve a value:

1# Using get() method
2print(my_dict.get('age'))  # Output: 25

1user =  {'name':  'William',  'age':  22}
2print(user.get('gender',  None))  # Output: None

If a key is not present in the dictionary, the get() method returns None.

Updating Values

To update the value of a key in a dictionary, you can simply assign a new value to the key:

1# Updating values
2my_dict['age'] = 22
3print(my_dict)  # Output: {'name': 'William', 'age': 22, 'city': 'New York'}

Adding a New Key-Value Pair

To add a new key-value pair to a dictionary, you can simply assign a value to a new key:

1# Adding a new key-value pair
2my_dict['gender'] = 'Male'
3print(my_dict)  # Output: {'name': 'William', 'age': 22, 'city': 'New York', 'gender': 'Male'}

Removing a Key-Value Pair

To remove a key-value pair from a dictionary, you can use the del keyword:

1# Removing a key-value pair
2del my_dict['city']
3print(my_dict)  # Output: {'name': 'William', 'age': 22, 'gender': 'Male'}

Dictionary Methods

Python dictionaries have several useful methods that can be used to manipulate and work with dictionaries. Some of the most common methods include:

• keys(): Returns a list of all the keys in the dictionary
• values(): Returns a list of all the values in the dictionary
• items(): Returns a list of tuples containing the key-value pairs in the dictionary
• clear(): Removes all the key-value pairs from the dictionary
• copy(): Returns a copy of the dictionary

1# Dictionary methods
2print(my_dict.keys())  # Output: dict_keys(['name', 'age', 'gender'])
3print(my_dict.values())  # Output: dict_values(['William', 22, 'Male'])
4print(my_dict.items())  # Output: dict_items([('name', 'William'), ('age', 22), ('gender', 'Male')])

Iterating Over a Dictionary You can use a for loop to iterate over the keys in a dictionary:

1# Iterating over a dictionary
2for key in my_dict:
3    print(key, my_dict[key])

Output:

1name William
2age 22
3gender Male