Strings Are Immutable

name = "alice"
result = name.upper()

print(result)    # New string
print(name)      # Original unchanged

To keep the result, you must assign it to a variable (either the same variable or a new one).

name = "alice"
name = name.upper()   # Reassign to same variable
print(name)

Case Conversion Methods

upper() and lower()

message = "Hello, World!"

print(message.upper())   # All uppercase
print(message.lower())   # All lowercase

title() and capitalize()

text = "the quick brown fox"

print(text.title())       # Capitalize Each Word
print(text.capitalize())   # Capitalize first letter only

user_input = "YES"

# Compare in lowercase so "YES", "Yes", "yes" all work
if user_input.lower() == "yes":
    print("User confirmed!")

swapcase()

text = "Hello World"
print(text.swapcase())

Stripping Whitespace

The strip() family of methods removes leading and/or trailing whitespace (spaces, tabs, newlines).

messy = "   Hello, World!   "

print(f"'{messy.strip()}'")    # Both sides
print(f"'{messy.lstrip()}'")   # Left side only
print(f"'{messy.rstrip()}'")   # Right side only

Stripping Specific Characters

You can also strip specific characters by passing them as an argument.

data = "###Price: $25.99###"
print(data.strip("#"))

url = "https://example.com/"
print(url.rstrip("/"))

The replace() Method

replace(old, new) returns a new string with all occurrences of old replaced by new.

sentence = "I like cats. Cats are great."
print(sentence.replace("cats", "dogs"))

Note that replace() is case-sensitive. "Cats" was not replaced because it starts with a capital C.

Limiting Replacements

You can limit how many replacements are made with a third argument.

text = "one fish, two fish, red fish, blue fish"
print(text.replace("fish", "bird", 2))   # Replace only first 2

# Clean up a phone number
phone = "(555) 123-4567"
clean = phone.replace("(", "").replace(")", "").replace("-", "").replace(" ", "")
print(clean)

The count() Method

count() returns the number of non-overlapping occurrences of a substring.

text = "banana"
print(text.count("a"))
print(text.count("an"))
print(text.count("z"))

paragraph = "Python is great. Python is popular. Python is fun."
print(f"'Python' appears {paragraph.count('Python')} times")
print(f"Sentences: {paragraph.count('.')}")

startswith() and endswith()

These methods return True or False based on whether a string starts or ends with a given substring.

filename = "report_2024.pdf"

print(filename.startswith("report"))
print(filename.endswith(".pdf"))
print(filename.endswith(".txt"))

files = ["photo.jpg", "report.pdf", "data.csv", "image.png", "notes.txt"]

images = [f for f in files if f.endswith((".jpg", ".png"))]
print("Image files:", images)

Checking Methods: isdigit(), isalpha(), isalnum()

print("123".isdigit())     # All digits?
print("hello".isalpha())   # All letters?
print("abc123".isalnum())  # All letters or digits?
print("hello".islower())   # All lowercase?
print("HELLO".isupper())   # All uppercase?