Unit 11.3A · Term 3

List & String Methods

Python provides powerful built-in methods for manipulating text (strings) and collections (lists). Mastering these tools allows you to write cleaner and more efficient code.

Learning Objectives

11.2.3.3 Apply functions and methods of processing lists
11.2.2.3 Apply functions and methods of processing strings

Lesson Presentation

11.3A-list-string-methods.pdf · Slides for classroom use

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Conceptual Anchor

The Power Tools Analogy

If basic operations are hand tools, methods are power tools. split() is a saw (cutting a log into planks), join() is glue, and sort() organizes your workshop instantly.

Rules & Theory

String Methods (11.2.2.3)

# Changing Case
s = "Hello World"
print(s.upper())       # "HELLO WORLD"
print(s.lower())       # "hello world"
print(s.title())       # "Hello World"
print(s.swapcase())    # "hELLO wORLD"

# Searching & Replacing
print(s.find("World")) # 6 (index where it starts)
print(s.replace("World", "Python")) # "Hello Python"
print(s.count("l"))    # 3

# Checking Content
print("123".isdigit()) # True
print("abc".isalpha()) # True
print(" ".isspace())   # True

# Splitting & Joining
words = s.split()      # ['Hello', 'World']
print("-".join(words)) # "Hello-World"

# Stripping Whitespace
raw = "   user input   "
print(raw.strip())     # "user input"

List Methods (11.2.3.3)

lst = [3, 1, 4, 1, 5]

# Adding Elements
lst.append(9)          # [3, 1, 4, 1, 5, 9] (add to end)
lst.insert(0, 2)       # [2, 3, 1, 4, 1, 5, 9] (insert at index 0)
lst.extend([6, 7])     # [2, 3, 1, 4, 1, 5, 9, 6, 7] (add another list)

# Removing Elements
lst.pop()              # Removes last element (7)
lst.pop(0)             # Removes element at index 0 (2)
lst.remove(4)          # Removes first occurrence of value 4
# lst.clear()          # Removes all elements

# Ordering
lst.sort()             # Sorts in-place: [1, 1, 3, 5, 6, 9]
print(lst)
lst.reverse()          # Reverses in-place: [9, 6, 5, 3, 1, 1]

# Information
print(lst.count(1))    # 2
print(lst.index(9))    # 0 (index of first 9)

In-place vs Return

String methods return a new string (strings are immutable). List methods like sort() change the list in-place and return None.

Worked Examples

1 Palindrome Checker

def is_palindrome(text):
    # Remove spaces and convert to lower case
    clean_text = text.replace(" ", "").lower()
    # Compare with reverse
    return clean_text == clean_text[::-1]

print(is_palindrome("Race car"))  # True
print(is_palindrome("Hello"))     # False

2 Processing a List of Names

names = ["  ali ", "DANA", "marat  "]
cleaned_names = []

for name in names:
    cleaned_names.append(name.strip().title())

cleaned_names.sort()
print(cleaned_names) # ['Ali', 'Dana', 'Marat']

Pitfalls & Common Errors

Sorting Mistake

x = lst.sort() sets x to None! Determine if you want to modify the list (lst.sort()) or create a new sorted list (x = sorted(lst)).

String Immutability

s.upper() does NOT change s. You must assign it: s = s.upper().

Graded Tasks

Remember

What method adds a single element to the end of a list?

Understand

Explain why string.replace("a", "b") doesn't verify until you print or assign it.

Apply

Write a program that takes a sentence, splits it into words, sorts them alphabetically, and joins them with hyphens.

Create

Create a password validator that checks if a string has at least one uppercase letter, one digit, and no spaces, using string methods.