Chapter 9: Tuples, Lists, and Dictionaries

Creating Lists

You can create lists using square brackets or the list() constructor:

# Empty list
empty_list = []

# List with items
numbers = [1, 2, 3, 4, 5]
fruits = ["apple", "banana", "orange"]
mixed = [1, "hello", 3.14, True]

List Indexing and Slicing

Access list elements using index starting from 0:

fruits = ["apple", "banana", "orange"]
print(fruits[0])  # "apple"
print(fruits[-1]) # "orange" (last element)

# Slicing
print(fruits[1:3]) # ["banana", "orange"]

Modifying Lists

• Change an item: fruits[1] = "grape"
• Add an item: fruits.append("kiwi")
• Insert at position: fruits.insert(1, "pear")
• Remove an item: fruits.remove("banana")
• Remove by index: del fruits[2]
• Pop last item: last_fruit = fruits.pop()

List Methods

Common list methods include:

• len(list): Get the length
• list.sort(): Sort the list in place
• list.reverse(): Reverse the list in place
• list.index(item): Find index of item
• list.count(item): Count occurrences of item

The sorted() Function

The sorted() function returns a new sorted list:

numbers = [3, 1, 4, 1, 5]
sorted_numbers = sorted(numbers)
print(sorted_numbers)  # [1, 1, 3, 4, 5]

What Are Tuples?

Tuples are immutable ordered sequences, similar to lists but cannot be modified after creation. They are defined using parentheses:

# Creating tuples
empty_tuple = ()
single_item = ("hello",)
coordinates = (3.14, 2.71, 1.41)
person = ("Alice", 30, "Engineer")

Tuple Immutability

Once created, tuple elements cannot be changed:

coordinates = (3.14, 2.71, 1.41)
# coordinates[0] = 3.0  # TypeError!

# But you can create new tuples
coordinates = (3.0,) + coordinates[1:]  # Creates new tuple

Tuple Unpacking

Multiple assignment works well with tuples:

name, age, profession = ("Alice", 30, "Engineer")
print(name, age, profession)  # Alice 30 Engineer

# Swapping values
a = 5
b = 10
a, b = b, a  # a=10, b=5

When to Use Tuples vs Lists

• Tuples: For data that should not change (immutable)
• Lists: For collections that need modification
• Lists: When you need features like append(), insert(), remove()
• Tuples: Can be used as dictionary keys (lists cannot)

List Comprehensions

Compact way to create new lists from existing ones:

# Create squares of numbers 1-5
squares = [x**2 for x in range(1, 6)]
print(squares)  # [1, 4, 9, 16, 25]

# With condition
even_squares = [x**2 for x in range(1, 11) if x % 2 == 0]
print(even_squares)  # [4, 16, 36, 64, 100]

Copying Lists

Shallow vs Deep Copy:

original = [1, 2, [3, 4]]

# Shallow copy - nested lists share references
shallow = original[:]
shallow[2][0] = 99
print(original)  # [1, 2, [99, 4]]

import copy
deep = copy.deepcopy(original)
deep[2][0] = 88
print(original)  # Still [1, 2, [99, 4]]

Creating Dictionaries

# Empty dictionary
empty_dict = {}

# Dictionary with items
person = {
    "name": "Alice",
    "age": 30,
    "profession": "Engineer"
}

# Using dict() constructor
coordinates = dict(x=10, y=20, z=30)

Accessing Dictionary Values

person = {"name": "Alice", "age": 30}

print(person["name"])  # Alice

# Using get() to avoid KeyError
print(person.get("salary", "Not specified"))  # Not specified

# Check if key exists
if "name" in person:
    print("Name found:", person["name"])

Modifying Dictionaries

person = {"name": "Alice", "age": 30}

# Add new key-value pair
person["profession"] = "Engineer"

# Update existing value
person["age"] = 31

# Remove items
del person["age"]
profession = person.pop("profession", "Not specified")

# Get all keys/values/items
keys = person.keys()
values = person.values()
items = person.items()

Common List Issues

• IndexError: Trying to access nonexistent index
• TypeError: Wrong operations on data types
• Modifying while iterating: Exception or unexpected behavior
• Shallow copy traps: Nested objects aren't copied deeply

Dictionary Debugging

• KeyError: Accessing non-existent key
• KeyError vs None: Different behaviors
• Mutable keys: Lists can't be dictionary keys
• Iteration behavior: Order of insertion preserved in Python 3.7+

# Debug dictionary operations
person = {"name": "Alice", "age": 30}

# Check keys before access
key = "salary"
if key in person:
    print(f"{key}: {person[key]}")
else:
    print(f"{key} not found")

# Use get() for safety
salary = person.get("salary", "Not available")
print(f"Salary: {salary}")

# Print dictionary contents for debugging
print("Person dict:", person)
print("Keys:", list(person.keys()))
print("Values:", list(person.values()))