Dictionary

A dictionary in Python is an unordered and mutable collection of key-value pairs. Dictionaries are defined using curly braces {}. Each item in a dictionary is a pair of a key and a corresponding value, separated by a colon :. Here’s an example of a dictionary:

my_dict = {
   'name': 'John Doe',
   'age': 30,
   'city': 'New York'
}

In this dictionary, 'name', 'age', and 'city' are keys, and 'John Doe', 30, and 'New York' are their corresponding values.

Accessing Items

You can access the items of a dictionary by referring to its key name:

print(my_dict['name'])  # outputs: John Doe

If you try to access a key that does not exist, Python will raise a KeyError. To avoid this, you can use the get method which will return None or a default value if the key does not exist:

print(my_dict.get('address'))  # outputs: None
print(my_dict.get('address', 'Unknown'))  # outputs: Unknown

Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it:

my_dict['address'] = '123 Main St'

Modifying Items

You can modify the value of a specific item by referring to its key name:

my_dict['city'] = 'Los Angeles'

Removing Items

You can remove items using the pop method or the del keyword:

my_dict.pop('address')  # removes the item with key 'address'
del my_dict['age']  # removes the item with key 'age'

Iterating over Items

You can iterate over the keys, values, or key-value pairs in a dictionary:

# Iterating over keys
for key in my_dict:
   print(key)
# Iterating over values
for value in my_dict.values():
   print(value)
# Iterating over key-value pairs
for key, value in my_dict.items():
   print(key, value)

Checking if Key Exists

You can check if a specific key is present in the dictionary:

if 'name' in my_dict:
   print('name is a key in my_dict')

Dictionary Comprehension

Dictionary comprehensions provide a concise way to create dictionaries. Here’s an example of creating a dictionary using dictionary comprehension:

squares = {x: x*x for x in range(6)}
print(squares)  # outputs: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Nested Dictionaries

You can have dictionaries inside dictionaries, which are known as nested dictionaries:

my_dict = {
   'person1': {
       'name': 'John Doe',
       'age': 30,
   },
   'person2': {
       'name': 'Jane Doe',
       'age': 25,
   }
}

Dictionary Length

To determine how many items (key-value pairs) a dictionary has, you can use the len() function:

print(len(my_dict))  # outputs: 2

Clearing All Items

You can remove all items in the dictionary using the clear() method:

my_dict.clear()

Dictionaries are a powerful and flexible way to structure data in Python, and they are used extensively in various types of Python development.