Week 08 · Basic Track

Week 8 — String Methods & Formatting

Strings are immutable sequences with a rich method library. Learn to slice, search, replace, split, join, and format text for real-world use.

Basic Track2 min readFree lesson

Level: Basic | Prerequisite: Week 7 (Data Structures)

Learning Goals

String Indexing and Slicing
Frequently Used String Methods (split, join, strip, replace, etc.)
Advanced f-strings (Alignment and Width)
Escape Characters (\n, \t)

1. String Basics

Strings are immutable. Indexing and slicing work the same as lists.

s = "Python"
print(s[0])         # P
print(s[-1])        # n
print(s[1:4])       # yth
print(len(s))       # 6

#s[0] = "X"  # TypeError — cannot be changed

2. Commonly used methods

Method	Function	Example
`lower()` / `upper()`	Uppercase/Lowercase	`"Hi".upper()` → `"HI"`
`strip()`	Remove whitespace from both sides	`" hi ".strip()` → `"hi"`
`lstrip()` / `rstrip()`	Remove Left/Right whitespace
`split(sep)`	Split into a list	`"a,b,c".split(",")` → `["a","b","c"]`
`join(seq)`	Join a list	`",".join(["a","b"])` → `"a,b"`
`replace(old, new)`	Replace	`"foo".replace("o", "")` → `"f*"`
`find(x)` / `index(x)`	Find position	`"hello".find("l")` → `2`
`count(x)`	Count	`"hello".count("l")` → `2`
`startswith(x)` / `endswith(x)`	Check Start/End	`"abc".endswith("c")` → `True`
`isdigit()` / `isalpha()`	Check Number/Letter

3. Advanced f-strings

name = "Alice"
score = 92.345

print(f"{name:<10}|")       #Left align width 10
print(f"{name:>10}|")       #Right align
print(f"{name:^10}|")       #Center
print(f"{name:*^10}|")      #Fill character * center

print(f"{score:.2f}")        #92.35  (two decimal places)
print(f"{1234567:,}")        #1,234,567  (thousands comma)
print(f"{0.85:.0%}")         # 85%
print(f"{255:b} {255:x}")    #Binary, Hexadecimal

4. Escape Characters

Notation	Meaning
`\n`	Newline
`\t`	Tab
`\\`	Backslash
`\"`	Double Quote
`\'`	Single Quote

print("Line1
Line2")        #Two lines
print("Name	Age")      #Tab separated
print("Path: C:\\dev")    # C:\dev

#raw string: ignore escape
print(r"C:\dev\new")      # C:\dev\new

5. Converting Between Strings and Lists

csv = "Apple,Banana,Grape"
fruits = csv.split(",")             #['Apple', 'Banana', 'Grape']

joined = " | ".join(fruits)         #"Apple | Banana | Grape"

Key Examples (examples/)

File	Content
`01_methods.py`	Commonly Used Methods
`02_split_join.py`	split / join Pattern
`03_fstring_format.py`	Advanced f-strings
`04_escape.py`	Escape Characters

⚠️ Common Mistakes

Trying to modify the original string

s = "hello"
s.replace("l", "L")    #Keep 's' as it is!
s = s.replace("l", "L")  #You need to reassign the result

If the split() argument is missing, the splitting standard "a b c".split() splits by space. "a,b".split() does not split.
find vs index If find is missing, -1 and index become ValueError.

Same quotes inside quotes without escape

s = "say "hi""    # SyntaxError
s = 'say "hi"'    # OK
s = "say \"hi\""  # OK

Printing braces in f-strings

print(f"{name}")     #Variable
print(f"{{name}}")   #Just print {name}

❓ FAQ

Q1. Does string length count Korean characters as one character? A. len() counts by code points, so usually 1 character. Emojis and combined characters are exceptions.

Q2. How to compare ignoring case? A. s1.lower() == s2.lower() or s1.casefold() == s2.casefold() (more accurately).

Q3. Are there multiple ways to insert variable values into a string? A. Old method: % , .format(). Currently recommended: f-string (fastest and most readable).

📝 Exercises (exercises/)

exercise1.md — Palindrome check
exercise2.md — Email format validation
exercise3.md — Text statistics (character count, word count, average length)

Next Week

Week 9. Function Basics — Code reuse

Example files

The runnable files used in this lesson. Copy them into your own project and experiment.

examples/01_methods.pyPython

s = "  Hello, Python!  "

print(s.strip())                   # "Hello, Python!"
print(s.lower())                   # "  hello, python!  "
print(s.upper())                   # "  HELLO, PYTHON!  "
print(s.replace("Python", "World"))
print(s.find("Python"))            # 9
print(s.count("l"))                # 3
print(s.startswith("  H"))         # True
print(s.endswith("!  "))           # True
print("123".isdigit())             # True
print("abc".isalpha())             # True

examples/02_split_join.pyPython

csv = "Apple,Banana,Grape"
fruits = csv.split(",")
print(fruits)

joined = " | ".join(fruits)
print(joined)

#Separated by spaces
s = "Hello   World    Python"
print(s.split())   # ['Hello', 'World', 'Python']

#Separated by line breaks
text = "Line 1
Line 2
Line 3"
print(text.splitlines())

examples/03_fstring_format.pyPython

name = "Alice"
score = 92.345
price = 1234567

print(f"{name:<10}|")    #Left align
print(f"{name:>10}|")    #Right align
print(f"{name:^10}|")    #Center
print(f"{name:*^10}|")   #Center, fill *

print(f"{score:.2f}")    # 92.35
print(f"{price:,}")      # 1,234,567
print(f"{0.85:.0%}")     # 85%
print(f"{255:b}")        #11111111 (binary)
print(f"{255:x}")        #ff (hexadecimal)

#Table format
data = [("Alice", 92), ("Bob", 78), ("Charlie", 100)]
for n, s in data:
    print(f"{n:<10}{s:>5} points")

examples/04_escape.pyPython

print("Line1
Line2
Line3")
print("Name	Age	Score")
print("Backslash: \")
print("Quote: "")
print('Inside single quote: '')

# raw string
print(r"C:\dev\new\test.py")

#Multiline
print("""Multiple lines
of text
are also possible""")

Assignment

Goal

Check if the input string reads the same forwards and backwards.

Requirements

Ignore spaces and letter case
If it is a palindrome, output "It is a palindrome", otherwise "It is not a palindrome"

Input/Output Example

String: Goose
It is a palindrome

String: A man a plan a canal Panama
It is a palindrome

String: hello
It is not a palindrome

Assignment 2. Email Format Validation

Goal

Simply check if an email address matches the format.

Validation Rules (Simple Version)

Must contain @ exactly once
. must appear at least once after @
No spaces

Input/Output Example

Email: [email protected]
Valid email

Email: foo@@bar.com
Incorrect format

Email: nobody
Incorrect format

Assignment 3. Text Statistics

Goal

Input a sentence and output the number of characters, number of words, and average word length.

Requirements

Character count excludes spaces
Word count is based on splitting by spaces
Average word length rounded to two decimal places

Input/Output Example

Sentence: Hello Python world

Number of characters (excluding spaces): 16
Number of words: 3
Average word length: 5.33

Show the reference solution

Try the assignment yourself first — comparing your attempt with a working solution is where most of the learning happens.

solution1.pyPython

s = input("String:")
cleaned = s.replace(" ", "").lower()

if cleaned == cleaned[::-1]:
    print("It is a palindrome")
else:
    print("It is not a palindrome")

solution2.pyPython

email = input("Email:").strip()

if " " in email:
    print("Incorrect format")
elif email.count("@") != 1:
    print("Incorrect format")
else:
    local, _, domain = email.partition("@")
    if "." not in domain or not local or not domain:
        print("Incorrect format")
    else:
        print("Valid email")

solution3.pyPython

text = input("Sentence:")

char_count = len(text.replace(" ", ""))
words = text.split()
word_count = len(words)
avg_len = sum(len(w) for w in words) / word_count if word_count else 0

print(f"Number of characters (excluding spaces): {char_count}")
print(f"Number of words: {word_count}")
print(f"Average word length: {avg_len:.2f}")

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