Python Dictionaries

Dictionaries in Python

Python dictionaries are powerful data structures used to store data values in key-value pairs. This lesson will explore the characteristics of dictionaries, how to create and manipulate them, and various examples demonstrating their functionality.

1. What is a Dictionary?

A dictionary is a collection that is ordered*, changeable, and does not allow duplicates. As of Python version 3.7, dictionaries maintain the order of items. In earlier versions, dictionaries were unordered.

Dictionaries are defined using curly brackets {} and consist of key-value pairs.

Example: Create and Print a Dictionary

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# Creating a dictionary
car_dict = {
  "brand": "Tesla",
  "model": "Model S",
  "year": 2020
}

print(car_dict)  # Output: {'brand': 'Tesla', 'model': 'Model S', 'year': 2020}

2. Dictionary Items

Dictionary items are presented in key-value pairs, and you can access them using the key names.

Example: Accessing a Dictionary Value

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# Creating a dictionary
car_dict = {
  "brand": "Tesla",
  "model": "Model S",
  "year": 2020
}

# Accessing the "model" value
print(car_dict["model"])  # Output: Model S

3. Ordered or Unordered?

As of Python 3.7, dictionaries are ordered collections. This means that items have a defined order, which will not change. In versions prior to 3.7, dictionaries were unordered.

Example: Checking Order

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# Creating a dictionary
color_dict = {
  "first": "red",
  "second": "green",
  "third": "blue"
}

print(color_dict)  # Output will show the order of items as defined

4. Changeable

Dictionaries are mutable, meaning you can change, add, or remove items after they have been created.

Example: Modifying a Dictionary

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# Creating a dictionary
person_dict = {
  "name": "Alice",
  "age": 30
}

# Changing the age
person_dict["age"] = 31
print(person_dict)  # Output: {'name': 'Alice', 'age': 31}

5. Duplicates Not Allowed

Dictionaries cannot have two items with the same key. If you use the same key again, the value will be overwritten.

Example: Duplicate Key Handling

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# Creating a dictionary with a duplicate key
product_dict = {
  "name": "Laptop",
  "price": 1000,
  "price": 1200  # This will overwrite the previous price
}

print(product_dict)  # Output: {'name': 'Laptop', 'price': 1200}

6. Dictionary Length

To determine how many items a dictionary contains, use the len() function.

Example: Get the Number of Items in a Dictionary

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# Creating a dictionary
item_dict = {
  "item1": "Book",
  "item2": "Pen",
  "item3": "Notebook"
}

print(len(item_dict))  # Output: 3

7. Dictionary Items - Data Types

The values in a dictionary can be of any data type, including strings, integers, booleans, and lists.

Example: Dictionary with Various Data Types

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# Creating a dictionary with mixed data types
mixed_dict = {
  "name": "Bob",
  "is_student": True,
  "age": 25,
  "courses": ["Math", "Science"]
}

print(mixed_dict)  # Output: {'name': 'Bob', 'is_student': True, 'age': 25, 'courses': ['Math', 'Science']}

8. What is the Data Type of a Dictionary?

From Python's perspective, dictionaries are defined as objects of the data type 'dict'.

Example: Print the Data Type of a Dictionary

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# Creating a dictionary
my_dict = {
  "brand": "Toyota",
  "model": "Camry",
  "year": 2022
}

print(type(my_dict))  # Output: <class 'dict'>