Lesson 12 — List Operations and Methods

Estimated time: 60 minutes

Learning Objectives

By the end of this lesson you will be able to:

  1. Add items with .append() and .insert()
  2. Remove items with .remove() and .pop()
  3. Search a list using the in operator and .index()
  4. Sort lists with .sort() and sorted()
  5. Build a dynamic list that grows as data is collected

Concepts

Adding Items

.append(value) — adds an item to the end of a list:

shopping = ["bread", "milk"]
shopping.append("eggs")
print(shopping)   # ["bread", "milk", "eggs"]

.insert(index, value) — adds an item at a specific position:

shopping.insert(1, "butter")  # Insert at index 1
print(shopping)   # ["bread", "butter", "milk", "eggs"]

Removing Items

.remove(value) — removes the first occurrence of a value:

shopping.remove("milk")   # Removes "milk"

⚠️ Raises ValueError if the item isn’t in the list! Check first with in.

.pop(index) — removes and returns the item at an index:

last = shopping.pop()       # Remove last item, return it
item = shopping.pop(0)      # Remove first item, return it

Searching

in operator — checks if a value is in a list:

fruits = ["apple", "banana", "mango"]
print("banana" in fruits)    # True
print("grape" in fruits)     # False

if "apple" in fruits:
    print("Apple is available!")

.index(value) — returns the index of the first occurrence:

print(fruits.index("banana"))   # 1

⚠️ Raises ValueError if not found! Check with in first.

.count(value) — counts how many times a value appears:

nums = [1, 2, 3, 2, 1, 2]
print(nums.count(2))   # 3

Sorting

.sort() — sorts the list in place (modifies the original):

scores = [45, 87, 23, 91, 56]
scores.sort()                  # Ascending
print(scores)   # [23, 45, 56, 87, 91]

scores.sort(reverse=True)      # Descending
print(scores)   # [91, 87, 56, 45, 23]

sorted(list) — returns a new sorted list, leaving the original unchanged:

original = [45, 87, 23]
sorted_copy = sorted(original)
print(original)      # [45, 87, 23]  — unchanged!
print(sorted_copy)   # [23, 45, 87]

Rule: Use .sort() when you want to modify the list in place. Use sorted() when you need to keep the original.

Building Lists Dynamically

Start with an empty list [], then use .append() as you go:

readings = []   # Start empty

for i in range(5):
    value = int(input(f"Enter reading {i+1}: "))
    readings.append(value)

print("All readings:", readings)
print("Average:", sum(readings) / len(readings))

Guided Walkthrough

Step 1: append, insert, remove, pop 

# Start with a list
shopping = ["apples", "bread", "milk"]
print("Start:", shopping)

shopping.append("eggs")
print("After append:", shopping)

shopping.insert(1, "butter")   # Insert at position 1
print("After insert:", shopping)

shopping.remove("bread")
print("After remove:", shopping)

removed = shopping.pop()        # Remove last
print(f"Popped: '{removed}'")
print("After pop:", shopping)

Step 2: Searching 

fruits = ["apple", "banana", "cherry", "apple", "mango"]

print("banana" in fruits)        # True
print("grape" in fruits)         # False
print(fruits.index("cherry"))    # 2
print(fruits.count("apple"))     # 2

# Safe search pattern
search = "cherry"
if search in fruits:
    pos = fruits.index(search)
    print(f"Found '{search}' at index {pos}")
else:
    print(f"'{search}' not in list")

Step 3: Sorting 

scores = [45, 87, 23, 91, 56, 78, 34]
print("Original:", scores)

sorted_scores = sorted(scores)    # New sorted copy
print("Sorted copy:", sorted_scores)
print("Original unchanged:", scores)

scores.sort(reverse=True)         # Sort in place, descending
print("In-place descending:", scores)

names = ["Charlie", "Alice", "Diana", "Bob"]
print("Names sorted:", sorted(names))   # Alphabetical

Step 4: Dynamic Sensor Reading Log

This simulates collecting readings and analysing them:

import machine, neopixel, time
import random

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

readings = []
print("Simulating 15 sensor readings...")

for i in range(15):
    # Simulate a reading (replace with actual sensor later)
    reading = random.randint(10, 100)
    readings.append(reading)
    print(f"Reading {i+1:2d}: {reading}")
    time.sleep(0.2)

print(f"\n--- Analysis of {len(readings)} readings ---")
print(f"Min:     {min(readings)}")
print(f"Max:     {max(readings)}")
print(f"Average: {sum(readings)/len(readings):.1f}")

sorted_readings = sorted(readings)
print(f"Sorted:  {sorted_readings}")

# Visual feedback
avg = sum(readings) / len(readings)
if avg > 70:
    np[0] = (255, 0, 0)    # Red — high
    print("HIGH average")
elif avg > 40:
    np[0] = (255, 150, 0)  # Orange — medium
    print("MEDIUM average")
else:
    np[0] = (0, 255, 0)    # Green — low
    print("LOW average")

np.write()
time.sleep(3)
np[0] = (0, 0, 0)
np.write()

Step 5: Top-5 High Scores List 

import random

high_scores = []

for round_num in range(10):
    score = random.randint(0, 100)
    print(f"Round {round_num+1}: scored {score}")

    # Insert in sorted order
    high_scores.append(score)
    high_scores.sort(reverse=True)

    # Keep only top 5
    if len(high_scores) > 5:
        high_scores.pop()   # Remove last (lowest)

    print(f"  Top scores so far: {high_scores}")

print("\nFinal top 5:", high_scores)

Challenges

⭐ Core

Ask the user to enter 5 numbers one at a time. Store them in a list. Then print: the sorted list (ascending), the maximum, the minimum, and whether the number 42 is in the list.

⭐⭐ Extension

Create a “to-do list” manager using a while True loop. Each iteration, ask the user to type “add”, “remove”, “show”, or “done”. Implement each command. When “done” is typed, print the final list and exit. Handle errors gracefully (e.g., trying to remove something that isn’t in the list).

⭐⭐⭐ Stretch

Simulate a leaderboard: run 20 “game rounds” with random scores (0–100). After each round, maintain a “top 5 leaderboard” list (sorted, max 5 items). If the new score makes the leaderboard, flash the LED green; otherwise red. After all 20 rounds, print the final leaderboard. Also track: how many times a score made the leaderboard, and the average of the final top-5.

Common Mistakes & Debugging

ValueError: list.remove(x): x not in list You tried to remove something that isn’t there. Use if value in my_list: to check first.

.sort() vs sorted() confusion .sort() modifies the original, returns None. Don’t write x = my_list.sort() — x will be None. Use x = sorted(my_list) if you need a new list.

max() on an empty list raises ValueError Check if len(readings) > 0: before calling max(), min(), or sum().

Forgetting that .pop() also returns the value x = my_list.pop() both removes the last item AND stores it in x. If you just want to delete: my_list.pop() without assignment is fine.

Key Vocabulary

Term Definition
append() Adds an item to the end of a list
insert() Adds an item at a specific index
remove() Removes the first occurrence of a value
pop() Removes and returns the item at a given index (default: last)
in operator Checks whether a value exists in a list — returns True or False
sort() Sorts a list in place (modifies the original)
sorted() Returns a new sorted list without changing the original
dynamic list A list that grows over time by appending items

Copyright © Paul Baumgarten.