Python Basics — String Methods

String Methods

Python strings come with dozens of built-in methods. Here are the most useful ones, organized by category.

Case Methods

text = "hello, World!"

print(text.upper())       # HELLO, WORLD!
print(text.lower())       # hello, world!
print(text.title())       # Hello, World!
print(text.capitalize())  # Hello, world!
print(text.swapcase())    # HELLO, wORLD!

casefold() — Aggressive Lowering

german = "Straße"
print(german.lower())     # straße
print(german.casefold())  # strasse  (better for comparisons)

Search Methods

find() and index()

text = "Hello, World!"

# find() returns -1 if not found
print(text.find("World"))     # 7
print(text.find("Python"))    # -1

# index() raises ValueError if not found
print(text.index("World"))    # 7
# print(text.index("Python"))  # ValueError

rfind() and rindex() — Search from Right

path = "/home/user/docs/file.txt"
print(path.rfind("/"))   # 15 (last occurrence)

count()

text = "banana"
print(text.count("a"))   # 3
print(text.count("na"))  # 2

startswith() and endswith()

filename = "report.pdf"

print(filename.startswith("report"))  # True
print(filename.endswith(".pdf"))      # True
print(filename.endswith((".pdf", ".doc", ".txt")))  # True (tuple of suffixes)

Modification Methods

Since strings are immutable, these all return new strings.

replace()

text = "Hello, World!"
print(text.replace("World", "Python"))  # Hello, Python!
print(text.replace("l", "L"))          # HeLLo, WorLd!
print(text.replace("l", "L", 1))       # HeLlo, World! (max 1 replacement)

strip(), lstrip(), rstrip()

Remove whitespace (or specified characters) from the edges:

text = "   Hello, World!   "

print(text.strip())     # "Hello, World!"
print(text.lstrip())    # "Hello, World!   "
print(text.rstrip())    # "   Hello, World!"

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

split()

Split a string into a list:

text = "apple banana cherry"
print(text.split())          # ['apple', 'banana', 'cherry']

csv_line = "Alice,25,NYC"
print(csv_line.split(","))   # ['Alice', '25', 'NYC']

# Limit splits
text = "one-two-three-four"
print(text.split("-", 2))    # ['one', 'two', 'three-four']

splitlines()

text = "Line 1\nLine 2\nLine 3"
print(text.splitlines())    # ['Line 1', 'Line 2', 'Line 3']

join()

Join a list into a string:

words = ["Hello", "World"]
print(" ".join(words))      # Hello World
print(", ".join(words))     # Hello, World
print("-".join(words))      # Hello-World

# Common pattern: join with newlines
lines = ["Line 1", "Line 2", "Line 3"]
print("\n".join(lines))

Validation Methods

These return True or False:

print("123".isdigit())      # True
print("abc".isalpha())      # True
print("abc123".isalnum())   # True
print("   ".isspace())      # True
print("Hello".istitle())    # True
print("hello".islower())    # True
print("HELLO".isupper())    # True

Practical: Input Validation

username = input("Enter username (letters and digits only): ")

if username.isalnum():
    print(f"Valid username: {username}")
else:
    print("Invalid! Only letters and digits allowed.")

Alignment Methods

text = "Hello"

print(text.center(20))       #        Hello
print(text.center(20, "-"))  # -------Hello--------
print(text.ljust(20, "."))   # Hello...............
print(text.rjust(20, "."))   # ...............Hello
print(text.zfill(10))        # 00000Hello

# zfill() is useful for numbers
print("42".zfill(5))    # 00042
print("-42".zfill(6))   # -00042 (handles sign correctly)

Other Useful Methods

expandtabs()

text = "Name\tAge\tCity"
print(text.expandtabs(12))
# Name        Age         City

maketrans() and translate()

table = str.maketrans("aeiou", "12345")
text = "hello world"
print(text.translate(table))  # h2ll4 w4rld

removeprefix() and removesuffix() (Python 3.9+)

filename = "test_report.py"
print(filename.removeprefix("test_"))   # report.py
print(filename.removesuffix(".py"))     # test_report

Complete Reference Table

Category	Method	Description
Case	upper()	All uppercase
	lower()	All lowercase
	title()	Title Case
	capitalize()	First char upper
	swapcase()	Swap case
	casefold()	Aggressive lowercase
Search	find(sub)	Index of first match (-1 if not found)
	rfind(sub)	Index of last match
	index(sub)	Like find but raises ValueError
	count(sub)	Count occurrences
	startswith(s)	Check prefix
	endswith(s)	Check suffix
Modify	replace(old, new)	Replace substring
	strip()	Remove edge whitespace
	lstrip() / rstrip()	Remove left/right whitespace
	split(sep)	Split into list
	join(list)	Join list into string
	splitlines()	Split by line breaks
Validate	isdigit()	All digits?
	isalpha()	All letters?
	isalnum()	All letters/digits?
	isspace()	All whitespace?
	islower() / isupper()	All lower/upper?
Align	center(w)	Center-align
	ljust(w) / rjust(w)	Left/right-align
	zfill(w)	Pad with zeros

Summary

• Python strings have 40+ built-in methods
• Case methods: upper(), lower(), title(), capitalize()
• Search: find(), count(), startswith(), endswith()
• Modify: replace(), strip(), split(), join()
• Validate: isdigit(), isalpha(), isalnum()
• All methods return new strings (strings are immutable)
• Chain methods: " hello ".strip().upper() → "HELLO"

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