Dictionaries

A dictionary in Python is a collection of key-value pairs. Each key maps to a specific value, and dictionaries are optimized for fast lookups.

Creating a Dictionary

You can create a dictionary using curly braces {} or the dict() constructor.

Example:

# Using curly braces
person = {"name": "Alice", "age": 25, "city": "New York"}

# Using dict()
person = dict(name="Alice", age=25, city="New York")

Accessing Values

You can access values by their keys.

Example:

print(person["name"])  # Output: Alice

Using get():

The get() method is safer because it does not raise an error if the key is missing.

print(person.get("name"))  # Output: Alice
print(person.get("job", "Not found"))  # Output: Not found

Modifying a Dictionary

Adding or Updating Key-Value Pairs:

person["job"] = "Engineer"  # Add a new key-value pair
person["age"] = 26          # Update an existing key-value pair

Removing Key-Value Pairs:

• pop(key): Removes a key and returns its value.
• popitem(): Removes and returns the last inserted key-value pair.
• del: Deletes a key-value pair.
• clear(): Empties the dictionary.

person.pop("age")          # Removes "age"
del person["city"]         # Removes "city"
person.clear()             # Empties the dictionary

Iterating Through Dictionaries

You can iterate through keys, values, or both.

Example:

person = {"name": "Alice", "age": 25, "city": "New York"}

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

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

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

Dictionary Methods

Nested Dictionaries

A dictionary can contain other dictionaries, enabling you to create complex data structures.

Example:

students = {
    "student1": {"name": "Alice", "age": 25},
    "student2": {"name": "Bob", "age": 22},
}

print(students["student1"]["name"])  # Output: Alice

Common Use Cases

1. Storing Mappings:

   capitals = {"France": "Paris", "Japan": "Tokyo", "India": "New Delhi"}
   

2. Counting Items:

   word_count = {}
   for word in ["apple", "banana", "apple"]:
       word_count[word] = word_count.get(word, 0) + 1
   print(word_count)  # Output: {'apple': 2, 'banana': 1}
   

3. Representing Objects:

   person = {"name": "Alice", "age": 25, "job": "Engineer"}
   

Practice Exercises

1. Create a dictionary person with the keys: "name", "age", and "city".
   • Access and print the "city" value.
   • Add a new key "job" with a value.
2. Write a function count_occurrences(items) that:
   • Takes a list as input.
   • Returns a dictionary where the keys are the items, and the values are the number of occurrences.
3. Create a nested dictionary classroom where:
   • Each key represents a student name.
   • Each value is another dictionary containing "age" and "grade".
   • Access and print the grade of a specific student.

Dictionaries are a powerful and versatile tool for organizing data in Python!