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Module 018: Lists: Basics

Phase: 2. Control Flow & Data | Estimated time: 2 hours | Milestone Project: No

Prerequisites

Learning Objectives

  • Create lists with square brackets and list()
  • Store heterogeneous elements in a list
  • Access elements with positive and negative indexing
  • Slice lists to extract sublists
  • Use len() to get the list length
  • Concatenate (+) and repeat (*) lists
  • Check membership with in
  • Iterate over lists with for loops
  • Create and access nested lists

Why This Matters

Lists are the most fundamental data structure in Python. Unlike strings (which are sequences of characters), lists can hold any type of data — numbers, strings, other lists, or a mix. They're mutable (changeable), flexible, and used everywhere: from storing user input to representing data sets and building complex data structures.

Concept Explanation

Creating Lists

Lists are created with square brackets []:

empty = []
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", 3.14, True]
nested = [[1, 2], [3, 4], [5, 6]]

You can also use the list() constructor:

chars = list("hello")
print(chars)  # ['h', 'e', 'l', 'l', 'o']

Heterogeneous Elements

Lists can hold different types in the same list:

info = ["Alice", 28, 5.6, True]
print(info)  # ['Alice', 28, 5.6, True]

Indexing (Positive and Negative)

Access elements by their position (0-indexed):

fruits = ["apple", "banana", "cherry", "date"]
print(fruits[0])   # apple
print(fruits[2])   # cherry
print(fruits[-1])  # date   (last element)
print(fruits[-2])  # cherry (second-to-last)

Positive index: 0 is first, length-1 is last. Negative index: -1 is last, -length is first.

Slicing

Extract a portion of a list with list[start:stop:step]:

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(nums[2:6])     # [2, 3, 4, 5]
print(nums[:4])      # [0, 1, 2, 3]     (start defaults to 0)
print(nums[6:])      # [6, 7, 8, 9]     (stop defaults to end)
print(nums[::2])     # [0, 2, 4, 6, 8]  (every 2nd)
print(nums[::-1])    # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]  (reversed)

Slicing returns a new list — it doesn't modify the original.

len() — List Length

colors = ["red", "green", "blue"]
print(len(colors))  # 3

Concatenation (+) and Repetition (*)

a = [1, 2, 3]
b = [4, 5, 6]
print(a + b)       # [1, 2, 3, 4, 5, 6]
print(a * 3)       # [1, 2, 3, 1, 2, 3, 1, 2, 3]
print([0] * 5)     # [0, 0, 0, 0, 0]

Membership with in

fruits = ["apple", "banana", "cherry"]
print("banana" in fruits)   # True
print("grape" not in fruits)  # True

Iterating with for

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit.upper())
# Output:
# APPLE
# BANANA
# CHERRY

Index iteration pattern:

fruits = ["apple", "banana", "cherry"]
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")
# Output:
# 0: apple
# 1: banana
# 2: cherry

Nested Lists

Lists can contain other lists:

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print(matrix[0])      # [1, 2, 3]     (first row)
print(matrix[1][2])   # 6             (row 1, column 2)

Iterating over nested lists:

for row in matrix:
    for cell in row:
        print(cell, end=" ")
    print()
# Output:
# 1 2 3
# 4 5 6
# 7 8 9

Common Pitfalls

  1. Index out of range — Accessing an index equal to or greater than len(list) raises IndexError. Always check len() first.

  2. Confusing negative indexing — -0 is the same as 0 (first element). There's no negative zero index.

  3. Shallow copies with slicing confusion — new_list = old_list[:] creates a shallow copy. For simple lists of numbers/strings, this is fine but be aware of it for nested lists.

  4. Modifying a list while iterating — Adding or removing elements during a for loop causes skipped items or IndexError. We'll cover safe patterns later.

  5. Using + to build large lists incrementally — Each + creates a new list. For large data, use .append() (covered in Module 019).

Hands-On Walkthrough

Let's build a simple grade tracker with lists:

grades = []
num_students = int(input("How many students? "))  # Assume 3

for i in range(num_students):
    grade = float(input(f"Enter grade for student {i + 1}: "))
    grades.append(grade)

print(f"Grades: {grades}")
print(f"Average: {sum(grades) / len(grades):.1f}")
print(f"Highest: {max(grades)}")
print(f"Lowest: {min(grades)}")

Now let's work with a tic-tac-toe board using nested lists:

board = [[" ", " ", " "],
         [" ", " ", " "],
         [" ", " ", " "]]

board[0][0] = "X"
board[1][1] = "O"
board[2][2] = "X"

for row in board:
    print("|" + "|".join(row) + "|")
# Output:
# |X| | |
# | |O| |
# | | |X|

Key Takeaways

  • Lists are created with [] and can hold any mix of data types
  • Indexing starts at 0; negative indices count from the end (-1 is last)
  • Slicing (list[start:stop:step]) creates a new sublist
  • len() returns the number of elements
  • + concatenates two lists; * repeats a list
  • in checks membership; for x in list: iterates
  • Lists are mutable — you can change elements with assignment: list[0] = new_value
  • Nested lists model 2D data like matrices and grids

Further Reading

Next Module

Module 019: Lists: Methods & Comprehensions — Learn powerful list methods and the elegant list comprehension syntax.