Strings Toolbox — upper, lower, replace, count

By the end of this lesson you can:

• Compare two strings case-insensitively using .lower() or .upper() — so "Yes", "YES" and "yes" all count as the same answer.
• Swap pieces of a string with .replace(old, new) and count occurrences with .count(sub).
• Explain in your own words why greeting.upper() does not change greeting — strings are immutable, and clean up messy input() with .strip().

Two lessons ago we hunted bugs in a list lab. Today we switch from lists of items to lists of letters — strings. Same dot-method idea (my_list.append(...), my_list.sort()) but on text.

Quick predict-the-output puzzle. What does Python print?

greeting = "Selamat Pagi"
shout = greeting.upper()

print(shout)
print(greeting)

SELAMAT PAGI
Selamat Pagi

The dot tells a string what to do

You've seen the dot before — menu.append("teh tarik"), numbers.sort(). Strings have their own family of dot-methods. The shape is always the same:

some_string.method_name(arguments)

Today's four headline tools:

• .upper() — give me a SHOUTING copy.
• .lower() — give me a quiet, lowercase copy.
• .replace(old, new) — give me a copy with old swapped for new.
• .count(sub) — tell me how many times sub appears.

upper and lower — case-blind comparisons

The classic bug: you ask the user a yes/no question, but they type "Yes" with a capital Y and your if answer == "yes" says no. Fix it by flattening the case before comparing:

answer = "Yes"

if answer.lower() == "yes":
    print("Locked in!")
else:
    print("No worries.")
# → Locked in!

Notice we did not change answer. We made a temporary lowercase copy just for the comparison. The original answer is untouched — handy when you still want to greet the user with their original capitalisation later.

replace — swap one piece for another

order = "teh tarik, teh tarik, kopi"
fixed = order.replace("teh tarik", "milo ais")

print(fixed)
# → milo ais, milo ais, kopi

Two things to spot. First, .replace swaps every match, not just the first. Second — same rule again — order itself is unchanged. We caught the new string in fixed.

count — how many times does X appear?

sentence = "kuih kuih kuih and more kuih"
how_many = sentence.count("kuih")

print(how_many)
# → 4

.count is great for quick word-frequency games and for sanity-checking input — "does this phone number have exactly ten digits?"-style checks.

The immutability rule (the big one)

Strings are immutable. That's a fancy word meaning "can't be changed in place". Compare:

# lists CAN change in place
dishes = ["satay", "rojak"]
dishes.append("cendol")
print(dishes)
# → ['satay', 'rojak', 'cendol']

# strings CANNOT
name = "aisyah"
name.upper()
print(name)
# → aisyah     (still lowercase!)

If you want the change to stick, catch the new string:

name = "aisyah"
name = name.upper()
print(name)
# → AISYAH

The right-hand side name.upper() builds a new string; the = sign reassigns the label name to point at the new one. Old "aisyah" is forgotten.

Bonus tool · strip() for messy input

When a user types into input(), they sometimes leave a stray space — especially on tablets. .strip() trims whitespace from both ends:

answer = "  yes  "
print(answer.strip())
# → yes

The everyday combo is .strip().lower() — clean, then flatten, then compare:

raw = input("Continue? ")
if raw.strip().lower() == "yes":
    print("Onwards!")

The job

Hafiz is writing a one-question quiz: "What is the capital of Malaysia?". The right answer is Kuala Lumpur, but classmates might type "kuala lumpur", "KUALA LUMPUR", "Kuala lumpur", or even " Kuala Lumpur " with stray spaces. All of those should count as correct.

Step 1 · Ask the question

raw = input("What is the capital of Malaysia? ")

Step 2 · Clean the input

Strip the spaces and flatten the case — chain the methods left-to-right:

clean = raw.strip().lower()

Read this aloud: "Take raw, strip the edges, then lowercase the result." Each dot-call returns a new string, and the next dot acts on that.

Step 3 · Compare against a clean target

Flatten the right-hand side too, so we never have to remember whether we wrote "kuala" or "Kuala":

if clean == "kuala lumpur":
    print("Correct!")
else:
    print("Not quite — the answer is Kuala Lumpur.")

Step 4 · Greet them with their original spelling

This is the bonus that immutability gives us for free — we still have raw in its original form:

print("You typed:", raw)

Putting it all together

Save as quiz_marker.py:

Code

# quiz_marker.py — case- and space-tolerant quiz

raw = input("What is the capital of Malaysia? ")
clean = raw.strip().lower()

if clean == "kuala lumpur":
    print("Correct!")
else:
    print("Not quite - the answer is Kuala Lumpur.")

print("You typed:", raw)

Sample run

What is the capital of Malaysia?   KUALA LUMPUR
Correct!
You typed:   KUALA LUMPUR

Three tasks today, each a bit harder. Type each one, run it, and only peek at the hint if you're truly stuck.

slogan = input("Type a slogan: ")
print(slogan)
print(slogan.upper())
print(slogan.lower())
print("Original:", slogan)

line = "Aisyah loves nasi lemak"
clean = line.lower()

print("a:", clean.count("a"))
print("e:", clean.count("e"))
print("i:", clean.count("i"))
print("o:", clean.count("o"))

review = "The kuih was awful, simply awful."
censored = review.replace("awful", "****")
hits = review.count("awful")

print(censored)
print("Words censored:", hits)

Strings have dot-methods just like lists do, but with one giant difference — they never change the original string, they return a brand-new one. .upper(), .lower() and .strip() are the trio for cleaning user input; .replace swaps pieces; .count tallies. If you want a change to stick, assign the result back with =.