Unit 11.2A · Term 2

Creating Lists & split/join

A list is Python's most versatile data structure — an ordered, mutable collection that can hold any type of data. The split() and join() methods provide powerful ways to convert between strings and lists.

Learning Objectives

11.2.3.1 Create lists in Python
11.2.3.2 Use the split and join methods

Lesson Presentation

11.2A-lesson-13-lists-create.pdf · Slides for classroom use

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Conceptual Anchor

The Shopping List Analogy

A list is like a shopping list: items are in a specific order (first item, second item...), you can add new items, remove items, or change an item (buy apples instead of oranges). split() is like tearing a long receipt into individual items. join() is like gluing them back together with a separator.

Rules & Theory

Creating Lists

# Method 1: Direct assignment
fruits = ["apple", "banana", "cherry"]
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", 3.14, True]

# Method 2: Empty list
empty = []
also_empty = list()

# Method 3: list() from other types
chars = list("Python")      # ['P', 'y', 't', 'h', 'o', 'n']
from_range = list(range(5)) # [0, 1, 2, 3, 4]
from_set = list({3, 1, 2})  # [1, 2, 3] (order may vary)

# Method 4: List repetition
zeros = [0] * 5             # [0, 0, 0, 0, 0]
pattern = [1, 2] * 3        # [1, 2, 1, 2, 1, 2]

# Method 5: List comprehension
squares = [x**2 for x in range(1, 6)]  # [1, 4, 9, 16, 25]
evens = [x for x in range(20) if x % 2 == 0]  # [0, 2, 4, ..., 18]

# Method 6: Building from input
n = int(input("How many? "))
my_list = []
for i in range(n):
    val = int(input(f"Value {i+1}: "))
    my_list.append(val)

split() — String → List

# Split by whitespace (default)
text = "Hello World Python"
words = text.split()
print(words)   # ['Hello', 'World', 'Python']

# Split by specific delimiter
date = "2025-02-19"
parts = date.split("-")
print(parts)   # ['2025', '02', '19']

csv = "Ali,95,A"
data = csv.split(",")
print(data)    # ['Ali', '95', 'A']

# Reading multiple numbers on one line
nums = input("Enter numbers: ").split()  # e.g., "10 20 30"
# nums = ['10', '20', '30'] — still strings!
nums = list(map(int, nums))  # [10, 20, 30] — now integers

join() — List → String

# Syntax: separator.join(list_of_strings)
words = ['Hello', 'World', 'Python']

print(" ".join(words))     # "Hello World Python"
print("-".join(words))     # "Hello-World-Python"
print(", ".join(words))    # "Hello, World, Python"
print("".join(words))      # "HelloWorldPython"

# join() only works with strings!
nums = [1, 2, 3]
# print("-".join(nums))   # TypeError!
print("-".join(str(n) for n in nums))  # "1-2-3" ✓

split() and join() are Inverses

"-".join("2025-02-19".split("-")) = "2025-02-19". They undo each other when using the same separator.

Worked Examples

1 Processing CSV Data

# Simulate reading a line from a CSV file
line = "Aisha,16,95.5,A"
data = line.split(",")

name = data[0]
age = int(data[1])
score = float(data[2])
grade = data[3]

print(f"Student: {name}, Age: {age}, Score: {score}, Grade: {grade}")

2 Reading Multiple Numbers from One Line

# User enters: 10 20 30 40 50
line = input("Enter numbers separated by spaces: ")
nums = list(map(int, line.split()))

print("Numbers:", nums)
print("Sum:", sum(nums))
print("Average:", sum(nums) / len(nums))

3 Reversing Words in a Sentence

sentence = "Python is awesome"
words = sentence.split()       # ['Python', 'is', 'awesome']
words.reverse()                 # ['awesome', 'is', 'Python']
result = " ".join(words)        # "awesome is Python"
print(result)

# One-liner version:
print(" ".join(sentence.split()[::-1]))

Pitfalls & Common Errors

split() Returns Strings

"10 20 30".split() gives ['10', '20', '30'] — strings, not integers! You must convert: list(map(int, ...)).

join() Requires All Strings

", ".join([1, 2, 3]) raises TypeError. Convert to strings first: ", ".join(str(x) for x in [1, 2, 3]).

Mutable Default Trap

a = [0] * 3 creates [0, 0, 0] — three separate integers (safe). But a = [[]] * 3 creates three references to the SAME inner list (dangerous!).

Pro-Tips for Exams

List Tips

split() without arguments splits by any whitespace and removes empty strings
split(",", 1) limits to 1 split: "a,b,c".split(",", 1) → ['a', 'b,c']
List comprehension is both faster and more Pythonic than a loop + append
To read N integers on one line: list(map(int, input().split()))

Graded Tasks

Remember

List 5 different ways to create a list in Python.

Understand

What is the difference between "hello world".split() and "hello world".split(" ")?

Apply

Write a program that reads a sentence and creates a list of word lengths (e.g., "Hello World" → [5, 5]).

Apply

Read 5 numbers on one line separated by commas. Split them into a list, convert to integers, and print their sum.

Analyze

What does list(range(10, 0, -2)) produce? What does [x**2 for x in range(5) if x % 2 != 0] produce?

Create

Build a "sentence scrambler" that takes a sentence, shuffles the word order randomly, and prints the scrambled version using split/join.

Self-Check Quiz

1. What does "a,b,c".split(",") return?

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2. What does '-'.join(['x', 'y', 'z']) return?

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3. What does [0] * 4 create?

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4. How do you read 3 integers from one line?

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5. What does [x*2 for x in range(4)] produce?

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