Python Dictionaries

 Python Dictionaries

What is a Dictionary?

A dictionary in Python is an unordered collection of key-value pairs. It's like a real-world dictionary where you look up a word (key) to find its definition (value).

What are the features of the dictionary in Python?

In Python, a dictionary is an unordered collection of items that stores data in key-value pairs. Here are the key features of dictionaries:

1. Key-Value Pairs: Each item in a dictionary consists of a key and a corresponding value. Keys must be unique and immutable (e.g., strings, numbers, tuples), while values can be of any data type.
2. Unordered: Dictionaries do not maintain the order of items. However, as of Python 3.7, dictionaries preserve the insertion order of keys.
3. Mutable: Dictionaries can be modified after creation. You can add, remove, or change items.
4. Dynamic Size: The size of a dictionary can grow or shrink as items are added or removed.
5. Fast Lookups: Dictionaries provide average-case time complexity of O(1) for lookups, insertions, and deletions due to their underlying hash table implementation.
6. Nested Dictionaries: Dictionaries can contain other dictionaries as values, allowing for more complex data structures.
7. Comprehensions: Python supports dictionary comprehensions, allowing for concise creation of dictionaries using a single line of code.
8. Methods: Dictionaries come with various built-in methods, such as:
   - dict.keys(): Returns a view object displaying a list of all the keys.
   - dict.values(): Returns a view object displaying a list of all the values.
   - dict.items(): Returns a view object displaying a list of key-value pairs.
   - dict.get(key): Retrieves the value for the specified key, returning None if the key does not exist.
   - dict.pop(key): Removes the specified key and returns its value.

Example of a Dictionary

Features:

1. Keys are unique: Dictionary keys must be unique. Having duplicate keys defeats the purpose of using a dictionary to store information.
2. Keys Must be immutable: Dictionary keys must be of an immutable type. Strings and numbers are the two most commonly used data types as dictionary keys.
3. Dictionaries are unordered: A dictionary contains key-value pairs but does not possess an order for the pairs. Thus, a more precise definition is that a dictionary is an unordered collection of key-value pairs. NOTE: In python 3.6x dictionaries have become an ordered collection.

Creating a Dictionary:

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

Accessing Values:

You can access values using their corresponding keys:

print(my_dict['name'])  # Output: Alice

Adding or Modifying Values:

my_dict['country'] = 'USA'  # Add a new key-value pair

my_dict['age'] = 31           # Modify an existing value

Common Dictionary Methods:

• keys(): Returns a list of all keys.
• values(): Returns a list of all values.
• items(): Returns a list of key-value ¹ pairs as tuples.

get(key, default): Returns the value for the key. If the key is not found, returns the default value.

pop(key): Removes the key-value pair and returns the value.