How to Create a Dictionary in Python

What is a dictionary in Python?

• An unordered collection of items is a dictionary.
• Each item is a key-value pair such that:
  • The key is unique.
  • The value is associated with that key and can be any data type.

Creating a Dictionary

1. Using Curly Braces ({})

The most common way to create a dictionary is by enclosing key-value pairs within curly braces.

# Example
my_dict = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}
print(my_dict)

2. Using the dict() Constructor

You can also create a dictionary using the built-in dict() constructor.

# Example
my_dict = dict(name="Alice", age=25, city="New York")
print(my_dict)

Note: When using dict(), keys must be valid identifiers (like variable names).

3. Creating an Empty Dictionary

You can create an empty dictionary to add key-value pairs later.

# Example
empty_dict = {}
print(empty_dict)  # Output: {}

Adding Key-Value Pairs

You can add a key-value pair by assigning a value to a new key.

my_dict = {}
my_dict["name"] = "Alice"
my_dict["age"] = 25
print(my_dict)  # Output: {'name': 'Alice', 'age': 25}

Accessing Values

You can access the value of a specific key using square brackets or the get() method.

# Using square brackets
print(my_dict["name"])  # Output: Alice

# Using get()
print(my_dict.get("age"))  # Output: 25

Note: If the key doesn’t exist, my_dict[key] raises a KeyError, whereas my_dict.get(key) returns None.

Common Dictionary Operations

1. Updating Values

You can update the value of an existing key.

my_dict["age"] = 30
print(my_dict)  # Output: {'name': 'Alice', 'age': 30}

2. Removing Items

Use del, pop(), or popitem() to remove items:

• del: Removes a specific key-value pair.

del my_dict["age"]
print(my_dict)  # Output: {'name': 'Alice'}

• pop(): Removes a key and returns its value.

age = my_dict.pop("age")
print(age)        # Output: 25
print(my_dict)    # Output: {'name': 'Alice'}

• popitem(): Removes the last inserted key-value pair.

item = my_dict.popitem()
print(item)  # Output: ('city', 'New York')

Iterating Through a Dictionary

You can loop through keys, values, or both:

# Loop through keys
for key in my_dict:
    print(key)

# Loop through values
for value in my_dict.values():
    print(value)

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

Dictionary Methods

Here are some useful dictionary methods:

• keys(): Returns all keys.
• values(): Returns all values.
• items(): Returns all key-value pairs.
• clear(): Removes all items.
• update(): Updates the dictionary with another dictionary.

my_dict.update({"gender": "female"})
print(my_dict)  # Output: {'name': 'Alice', 'age': 25, 'gender': 'female'}

Nested Dictionaries

Dictionaries can contain other dictionaries.

nested_dict = {
    "person1": {"name": "Alice", "age": 25},
    "person2": {"name": "Bob", "age": 30}
}
print(nested_dict["person1"]["name"])  # Output: Alice

Key Characteristics of Dictionaries

• Keys must be immutable (e.g., strings, numbers, tuples).
• Values can be of any data type.
• Dictionaries are unordered (Python 3.6+ maintains insertion order).

Complete Example

# Create a dictionary
student = {
    "name": "John",
    "age": 20,
    "courses": ["Math", "Science"]
}

# Access values
print(student["name"])  # Output: John

# Add a new key-value pair
student["grade"] = "A"

# Update an existing key
student["age"] = 21

# Remove a key-value pair
del student["grade"]

# Loop through dictionary
for key, value in student.items():
    print(f"{key}: {value}")