Strings

Creating, formatting, and manipulating text with Python's built-in string methods.

Overview

Strings in Python are sequences of characters enclosed in single quotes ('hello'), double quotes ("hello"), or triple quotes ("""multi-line text"""). Unlike some languages, Python treats single and double quotes identically for regular strings choose whichever avoids the need to escape the other. Strings are immutable once created, you cannot change individual characters in place. Operations that appear to modify a string actually return a new string. Python's string type comes with a rich set of built-in methods (upper(), lower(), strip(), split(), replace(), and many more) and supports powerful formatting via f-strings (formatted string literals, introduced in Python 3.6). f-strings let you embed expressions directly inside string literals using {expression} syntax, making them the preferred way to build strings from variables.

Key Points

•Single and double quotes are interchangeable: "hello" and 'hello' are the same string
•Triple quotes (""" or ''') allow multi-line strings and embedded quotes without escaping
•Strings are immutable methods return new strings, they never modify the original
•f-strings: f"Hello, {name}!" embeds expressions inline the preferred modern formatting style
•len(s) returns the number of characters; s[0] accesses the first character; s[-1] the last
•Slicing: s[start:stop:step] extracts a substring (stop is exclusive)
•The in operator checks for substrings: "ell" in "hello" → True

Examples

String creation and basic operations

greeting = "Hello, World!"
print(len(greeting))        # 13
print(greeting[0])          # H
print(greeting[-1])         # !
print(greeting[7:12])       # World
print(greeting.lower())     # hello, world!
print(greeting.upper())     # HELLO, WORLD!

f-strings the modern way to format strings

name = "Alice"
age = 30
height = 1.75

print(f"My name is {name} and I am {age} years old.")
print(f"Height: {height:.2f} meters")      # format to 2 decimal places
print(f"Next year I will be {age + 1}")    # expressions work inside {}

f-strings (Python 3.6+) are preferred over older .format() and % formatting.

Useful string methods

s = "  hello, world!  "
print(s.strip())             # "hello, world!"  (removes leading/trailing whitespace)
print(s.strip().title())     # "Hello, World!"
print(s.strip().replace("world", "Python"))  # "hello, Python!"

csv = "apple,banana,cherry"
parts = csv.split(",")
print(parts)                 # ['apple', 'banana', 'cherry']
print(", ".join(parts))      # "apple, banana, cherry"

Common Mistakes

Trying to modify a character in place: s[0] = "H" raises TypeError because strings are immutable
Forgetting that string indices start at 0, not 1
Using + to concatenate many strings in a loop (slow) prefer join() or f-strings for efficiency

Note

To check all methods available on a string, type `dir("")` in the Python REPL. To read the docs for any method, use `help("".strip)` right in the REPL.

Strings

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What does `"hello"[-1]` return?