Dictionaries: Python's Key-Value Store

Introduction to Dictionaries

Dictionaries are a fundamental data structure in Python, providing a way to store collections of data in key-value pairs. They are incredibly versatile and efficient for retrieving information when you know a specific identifier (the key). In many other programming languages, this data structure is known as a hash map or associative array.

What are Key-Value Pairs?

Imagine you have a list of players and their corresponding jersey numbers. You can easily look up a player’s number if you know their name. This is the basic idea behind key-value pairs.

• Key: The unique identifier (e.g., player’s name, a product ID, a street number). Keys must be immutable – meaning their value cannot be changed after creation. Common key types include strings, numbers, and tuples.
• Value: The data associated with the key (e.g., jersey number, product details, player’s name). Values can be of any data type – strings, numbers, lists, even other dictionaries!

Creating Dictionaries

There are several ways to create dictionaries in Python:

1. Using Curly Braces {}:

This is the most common and straightforward method. You define the dictionary with curly braces, and inside, you list the key-value pairs separated by colons :.

player_info = {
    "Sachin": "Batsman",
    "Dravid": "Batsman",
    "Sehwag": "Batsman"
}
print(player_info) # Output: {'Sachin': 'Batsman', 'Dravid': 'Batsman', 'Sehwag': 'Batsman'}

2. Using the dict() Constructor:

The dict() constructor can be used in a couple of ways:

• From keyword arguments: This is suitable when your keys are valid Python identifiers (variable names).

  player_info = dict(Sachin="Batsman", Dravid="Batsman", Sehwag="Batsman")
  print(player_info) # Output: {'Sachin': 'Batsman', 'Dravid': 'Batsman', 'Sehwag': 'Batsman'}
  

• From a list of tuples: Each tuple contains a key-value pair.

  player_list = [("Sachin", "Batsman"), ("Dravid", "Batsman"), ("Sehwag", "Batsman")]
  player_info = dict(player_list)
  print(player_info) # Output: {'Sachin': 'Batsman', 'Dravid': 'Batsman', 'Sehwag': 'Batsman'}
  

3. Empty Dictionary:

You can create an empty dictionary and populate it later.

player_info = {}
player_info["Sachin"] = "Batsman"
player_info["Dravid"] = "Batsman"
print(player_info)

Accessing Values

You access values in a dictionary using their corresponding keys within square brackets [].

player_info = {"Sachin": "Batsman", "Dravid": "Batsman", "Sehwag": "Batsman"}

sachin_role = player_info["Sachin"]
print(sachin_role) # Output: Batsman

Important: If you try to access a key that doesn’t exist, a KeyError will be raised.

Safe Access with get()

The get() method provides a safer way to access values. It returns the value associated with the key if it exists, and a default value (which you can specify) if the key doesn’t exist.

player_info = {"Sachin": "Batsman", "Dravid": "Batsman"}

kohli_role = player_info.get("Kohli", "Unknown")  # Returns "Unknown" if "Kohli" is not a key
print(kohli_role) # Output: Unknown

sachin_role = player_info.get("Sachin")
print(sachin_role) # Output: Batsman

Modifying and Deleting Entries

• Adding/Updating: You can add new key-value pairs or update existing values using the assignment operator.

  player_info = {"Sachin": "Batsman"}
  player_info["Dravid"] = "Batsman" # Adds Dravid
  player_info["Sachin"] = "Captain" # Updates Sachin's role
  print(player_info)
  

• Deleting: Use the del keyword or the pop() method to remove entries.

  player_info = {"Sachin": "Batsman", "Dravid": "Batsman"}
  
  del player_info["Dravid"] # Deletes Dravid
  print(player_info)
  
  sachin_role = player_info.pop("Sachin") #Deletes and returns Sachin's role.
  print(sachin_role)
  

Checking for Key Existence

Use the in operator to check if a key exists in the dictionary.

player_info = {"Sachin": "Batsman", "Dravid": "Batsman"}

if "Sachin" in player_info:
    print("Sachin is in the dictionary")

if "Kohli" not in player_info:
    print("Kohli is not in the dictionary")

Dictionary Iteration

You can iterate through the keys, values, or key-value pairs of a dictionary.

player_info = {"Sachin": "Batsman", "Dravid": "Batsman", "Sehwag": "Batsman"}

# Iterate through keys
for key in player_info:
    print(key)

# Iterate through values
for value in player_info.values():
    print(value)

# Iterate through key-value pairs
for key, value in player_info.items():
    print(f"Key: {key}, Value: {value}")

Use Cases

Dictionaries are incredibly useful in various scenarios:

• Storing configurations: Representing application settings.
• Counting occurrences: Tracking the frequency of items in a list.
• Implementing caches: Storing frequently accessed data for faster retrieval.
• Representing relationships: Modeling data with key-value associations.