Week 8 — String Methods & Formatting

What you'll learn

• 1Use upper, lower, strip, replace, split, join
• 2Format strings with f-strings and format()
• 3Slice strings like lists
• 4Check membership with in and startswith/endswith
• 5Understand string immutability

1. String Basics

s = "  Hello, World!  "
print(s.upper())         # "  HELLO, WORLD!  "
print(s.lower())         # "  hello, world!  "
print(s.strip())         # "Hello, World!"
print(s.strip().replace("World", "Python"))
print(len(s))            # 18

2. Split & Join

csv = "Alice,Bob,Charlie,Diana"
names = csv.split(",")    # list of strings
print(names)              # ['Alice', 'Bob', ...]
joined = " | ".join(names)
print(joined)             # Alice | Bob | Charlie | Diana

3. String Formatting

name, score = "Alice", 92.5
# f-string (recommended)
print(f"{name} scored {score:.1f}")
# format()
print("{} scored {:.1f}".format(name, score))
# Width & alignment
print(f"{name:<10}|{score:>8.2f}")  # left|right aligned

4. Useful Methods

5. Common Mistakes

1. Forgetting that strings are immutable: s.upper() doesn't change s — you must reassign: s = s.upper().
2. split() with no argument splits on any whitespace and removes empty strings; split(' ') preserves them.
3. "hello"[0] = 'H' → TypeError. Create a new string: 'H' + "hello"[1:].

💻 Examples

Run these examples and check the output yourself.

raw = "  python is AWESOME!  "
cleaned = raw.strip().lower()
print(cleaned)
print(cleaned.count("is"))
print(cleaned.replace("python", "coding"))

python is awesome!
1
coding is awesome!

line = "Alice,25,Seoul,Developer"
fields = line.split(",")
labels = ["Name", "Age", "City", "Job"]
for label, value in zip(labels, fields):
    print(f"{label:6}: {value}")

Name  : Alice
Age   : 25
City  : Seoul
Job   : Developer

students = [("Alice", 95), ("Bob", 82), ("Charlie", 71)]
print(f"{'Name':<10} {'Score':>6}")
print("-" * 18)
for name, score in students:
    print(f"{name:<10} {score:>6}")

Name        Score
------------------
Alice           95
Bob             82
Charlie         71

📝 Exercises

Try them yourself first, then open the solution to compare.

Password: hello
Failed: too short
Failed: no digit
Failed: no uppercase

pw = input("Password: ")
if len(pw) < 8:
    print("Failed: too short")
if not any(c.isdigit() for c in pw):
    print("Failed: no digit")
if not any(c.isupper() for c in pw):
    print("Failed: no uppercase")
if len(pw) >= 8 and any(c.isdigit() for c in pw) and any(c.isupper() for c in pw):
    print("Password OK")

Input: Hello World Python
Output: olleH dlroW nohtyP

sentence = input('Input: ')
result = ' '.join(w[::-1] for w in sentence.split())
print('Output:', result)