Unit 11.2A · Term 2

String Methods

Python strings come with powerful built-in methods — functions attached to string objects that let you search, modify, check, and transform text without writing complex loops.

Learning Objectives

11.2.2.3 Apply string methods

Lesson Presentation

11.2A-lesson-12-string-methods.pdf · Slides for classroom use

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Conceptual Anchor

The Swiss Army Knife

String methods are like a Swiss Army knife for text. Each blade (method) serves a specific purpose: one for converting case, another for searching, another for trimming spaces. Since strings are immutable, every method returns a new string — the original stays unchanged.

Rules & Theory

Case Methods

Method	Description	Example	Result
`.upper()`	All uppercase	`"hello".upper()`	`"HELLO"`
`.lower()`	All lowercase	`"HELLO".lower()`	`"hello"`
`.title()`	Title Case	`"hello world".title()`	`"Hello World"`
`.capitalize()`	First char upper	`"hello world".capitalize()`	`"Hello world"`
`.swapcase()`	Switch case	`"Hello".swapcase()`	`"hELLO"`

Search & Replace Methods

Method	Description	Example	Result
`.find(sub)`	Index of first occurrence (-1 if not found)	`"hello".find("ll")`	`2`
`.index(sub)`	Like find() but raises ValueError	`"hello".index("ll")`	`2`
`.count(sub)`	Count occurrences	`"banana".count("a")`	`3`
`.replace(old, new)`	Replace all occurrences	`"hello".replace("l", "r")`	`"herro"`
`.startswith(s)`	Starts with prefix?	`"hello".startswith("he")`	`True`
`.endswith(s)`	Ends with suffix?	`"hello.py".endswith(".py")`	`True`

Whitespace & Checking Methods

Method	Description	Example	Result
`.strip()`	Remove leading/trailing whitespace	`" hi ".strip()`	`"hi"`
`.lstrip()`	Remove leading whitespace	`" hi ".lstrip()`	`"hi "`
`.rstrip()`	Remove trailing whitespace	`" hi ".rstrip()`	`" hi"`
`.isdigit()`	All characters are digits?	`"123".isdigit()`	`True`
`.isalpha()`	All characters are letters?	`"abc".isalpha()`	`True`
`.isalnum()`	All alphanumeric?	`"abc123".isalnum()`	`True`
`.isupper()`	All uppercase?	`"ABC".isupper()`	`True`
`.islower()`	All lowercase?	`"abc".islower()`	`True`

Methods Return NEW Strings!

s.upper() does NOT change s. You must assign the result: s = s.upper(). This is because strings are immutable.

Worked Examples

1 Input Validation

# Clean and validate user input
name = input("Enter your name: ").strip().title()
age = input("Enter your age: ").strip()

if not age.isdigit():
    print("Error: age must be a number!")
else:
    print(f"Hello, {name}! You are {age} years old.")

2 Word Counter

sentence = input("Enter a sentence: ")
word = input("Search for word: ").lower()

# Case-insensitive count
count = sentence.lower().count(word)
print(f"'{word}' appears {count} time(s)")

# Find the position
pos = sentence.lower().find(word)
if pos != -1:
    print(f"First occurrence at index {pos}")
else:
    print("Word not found")

3 Censoring Words

text = "Python is awesome and Python is fun"
censored = text.replace("Python", "***")
print(censored)
# "*** is awesome and *** is fun"

# Replace only first occurrence
censored2 = text.replace("Python", "***", 1)
print(censored2)
# "*** is awesome and Python is fun"

4 File Extension Checker

filename = input("Enter filename: ")

if filename.endswith(".py"):
    print("Python file")
elif filename.endswith((".jpg", ".png", ".gif")):
    print("Image file")
elif filename.endswith(".txt"):
    print("Text file")
else:
    print("Unknown file type")

Pitfalls & Common Errors

Forgetting to Assign the Result

name.upper() alone doesn't change name. You must write name = name.upper().

find() vs index()

find() returns -1 if not found. index() raises ValueError if not found. Use find() when the substring might be absent.

Case Sensitivity

"Hello".find("hello") returns -1 because searches are case-sensitive. Convert both to lowercase first for case-insensitive search.

Pro-Tips for Exams

Method Chaining

You can chain methods: " Hello ".strip().lower() → "hello"
find() returns -1 (not False), so always compare with != -1
replace() replaces ALL occurrences by default. Add 3rd argument to limit.
isdigit() returns False for empty strings and negative numbers
Always clean input with .strip() — users often add accidental spaces

Graded Tasks

Remember

Name 5 string methods and describe what each does.

Understand

Explain the difference between find() and index().

Apply

Write a program that counts the number of vowels, consonants, digits, and spaces in a user-input string.

Apply

Write a program that validates a username: must be 3-15 characters, alphanumeric only, no spaces.

Analyze

What is the output of: s = " Hello World "; print(s.strip().replace("World", "Python").lower())?

Create

Build a simple text encryptor: shift each letter by 1 position (a→b, z→a). Use ord() and chr().

Self-Check Quiz

1. What does " hello ".strip() return?

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2. What does "banana".count("an") return?

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3. Does "hello".upper() change the original string?

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4. What does "hello".find("xyz") return?

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5. What does "123abc".isdigit() return?

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