Lesson 4: String Methods & Formatting

text = "  Hello, Python World!  "
 
# Case methods
print(text.upper())       # "  HELLO, PYTHON WORLD!  "
print(text.lower())       # "  hello, python world!  "
print(text.title())       # "  Hello, Python World!  "
print(text.capitalize())  # "  hello, python world!  " (only first char)
print(text.swapcase())    # "  hELLO, pYTHON wORLD!  "
 
# Whitespace methods
print(text.strip())       # "Hello, Python World!"
print(text.lstrip())      # "Hello, Python World!  "
print(text.rstrip())      # "  Hello, Python World!"
 
# Search methods
print(text.find("Python"))      # 9  (index of first match, -1 if not found)
print(text.index("Python"))     # 9  (like find but raises ValueError if not found)
print(text.count("l"))          # 3  (how many times "l" appears)
print(text.startswith("  H"))   # True
print(text.endswith("!  "))     # True
print("Python" in text)         # True (check if substring exists)
 
# Modify methods
print(text.replace("Python", "Programming"))  # replace all occurrences
print(text.replace("l", "L", 1))             # replace only first occurrence
 
# Split and join
sentence = "one two three four five"
words = sentence.split()          # ["one", "two", "three", "four", "five"]
words_by_comma = "a,b,c,d".split(",")  # ["a", "b", "c", "d"]
joined = " | ".join(words)        # "one | two | three | four | five"
 
# Check methods (return True/False)
print("hello".isalpha())      # True  (only letters)
print("hello123".isalpha())   # False
print("123".isdigit())        # True  (only digits)
print("hello".isalower())     # True  (all lowercase)
print("   ".isspace())        # True  (only whitespace)

# Real-world: clean and validate username input
raw_input = "  Alice_Johnson_2024  "
username = raw_input.strip().lower()
print(username)  # "alice_johnson_2024"
 
# Check username is valid (letters, numbers, underscores only)
# (We'll do the if/else check in Course 3)
is_valid = username.replace("_", "").isalnum()
print(f"Username '{username}' is valid: {is_valid}")

name = "Alice"
age = 28
salary = 85432.50
pi = 3.14159265
 
# Basic substitution
print(f"Hello, {name}!")                     # Hello, Alice!
 
# Expressions inside f-strings
print(f"In 10 years: {age + 10}")            # In 10 years: 38
print(f"Name length: {len(name)}")            # Name length: 5
print(f"Upper: {name.upper()}")              # Upper: ALICE
 
# Number formatting
print(f"Salary: ${salary:,.2f}")             # Salary: $85,432.50
print(f"Pi: {pi:.4f}")                       # Pi: 3.1416
print(f"Scientific: {salary:.2e}")           # Scientific: 8.54e+04
print(f"Percent: {0.756:.1%}")               # Percent: 75.6%
 
# Width and alignment
print(f"{'Name':<15}{'Age':>5}")             # Name              Age
print(f"{'Alice':<15}{28:>5}")               # Alice              28
print(f"{'Bob':<15}{32:>5}")                 # Bob                32
 
# Padding with specific character
print(f"{'Hello':*^30}")                     # ********Hello*********
print(f"{'Hello':->30}")                     # -------------------------Hello
print(f"{'Hello':-<30}")                     # Hello-------------------------
 
# Debug mode (Python 3.8+)
x = 42
print(f"{x=}")   # x=42  (shows variable name and value)

# Building a formatted table
students = [
    ("Alice Johnson", 22, 3.9),
    ("Bob Smith", 20, 3.5),
    ("Carol Williams", 25, 3.8),
    ("David Brown", 21, 3.2),
]
 
print(f"\n{'Name':<20} {'Age':>5} {'GPA':>6}")
print("-" * 33)
for name, age, gpa in students:   # We'll learn loops properly in Course 3
    print(f"{name:<20} {age:>5} {gpa:>6.1f}")

name = "Alice"
age = 28
 
# .format() method (Python 2.6+)
print("Hello, {}!".format(name))          # Hello, Alice!
print("Name: {0}, Age: {1}".format(name, age))  # Name: Alice, Age: 28
print("Name: {n}, Age: {a}".format(n=name, a=age))  # Named args
 
# % formatting (old style, from C)
print("Hello, %s!" % name)           # Hello, Alice!
print("Age: %d" % age)               # Age: 28
print("Pi: %.2f" % 3.14159)          # Pi: 3.14
print("Name: %s, Age: %d" % (name, age))
 
# Format specifiers comparison:
# f-string: f"{value:.2f}"
# .format(): "{:.2f}".format(value)
# % style:  "%.2f" % value

# You might see this in documentation or old tutorials:
template = "Dear {name},\n\nYour order #{order_id} for ${amount:.2f} has shipped.\n"
message = template.format(name="Alice", order_id=12345, amount=89.99)
print(message)