Strings - Classroom Notes

What is a String?

A string is a sequence of characters, like a word or a sentence.
"hello world"

How to Create a String

You can use single or double quotes: s1 = 'apple' s2 = "banana"

Tip: Quotes must match!

Strings are Immutable

You can't change a string after it's created!

s = "cat"
s[0] = "b" # ❌ Error!

To "change" a string, make a new one.

Indexing & Slicing

s = "python"
s[0] # 'p' s[2:5] # 'tho'

Indexes start at 0. Slicing is [start:end] (end not included).

Concatenation

greet = "Hi, " + "Alice!" # 'Hi, Alice!'

Common String Methods

s = "hello"
s.upper() # 'HELLO' s.replace("l", "r") # 'herro' s.split("e") # ['h', 'llo']

Try: lower(), strip(), find(), join(), ...

Fun Fact

Strings can be empty: ""

An empty string has length 0!

String Literals vs Variables

'hello' is a literal greet = 'hello' is a variable

A literal is a value; a variable stores it for reuse.

Escape Characters

s = 'It\'s sunny!' newline = "Line1\nLine2"

Use \n for new line, \t for tab, \\ for backslash, \' or \" for quotes.

Multi-line Strings

msg = """Hello\nWorld!"""

Triple quotes let you write strings over multiple lines.

String Formatting

name = "Sam"
"Hello, %s!" % name # 'Hello, Sam!' "Hello, {}!".format(name) # 'Hello, Sam!' f"Hello, {name}!" # 'Hello, Sam!'

f-strings (since Python 3.6) are the most modern and readable!

Membership & Iteration

'a' in "cat" # True for ch in "dog": print(ch)

You can check if a substring exists, or loop through each character.

Length of a String

len("banana") # 6

Use len() to count characters (including spaces and symbols).

Common Pitfalls

s = "123"
n = s + 1 # ❌ Error!

You can't add a string and a number directly. Use int(s) or str(n) to convert.

s = "hello"
s[0] = 'y' # ❌ Error!

Strings are immutable—use s = 'y' + s[1:] to "change" the first letter.

Real-World Analogy

Each bead is a character. You can count, pick, or slice beads, but can't change one in place!