Section · 01

Three ways to write a string

Strings hold text. Three legal ways to write them:

single = 'York'
double = "York"
triple = """York
spans
multiple lines"""

Single and double quotes do the same thing. Pick double quotes by default; switch to single quotes when your string contains a double quote (or vice versa) so you don’t have to escape:

message = "Don't quote me on that."     # easy
message = 'Don\'t quote me on that.'    # works but ugly
message = 'She said "hello".'            # easy
message = "She said \"hello\"."          # works but ugly

Triple-quoted strings are for multi-line text — usually for docstrings (the documentation at the top of a function) or templated chunks of HTML/SQL/JSON.

Escape sequences

A backslash starts an “escape sequence” inside a string. Three you’ll see constantly:

"line one\nline two"     # \n is a newline
"col1\tcol2"            # \t is a tab
"path: C:\\Users\\York"   # \\ is a literal backslash

If you have a string with lots of backslashes (Windows paths, regular expressions), prefix with rfor “raw string” and Python will leave the backslashes alone:

path = r"C:\Users\York\Documents"   # no escaping needed

Section · 02

Combining and formatting

You can glue strings together with +:

first = "Ada"
last = "Lovelace"
full = first + " " + last          # "Ada Lovelace"

This works but reads badly the moment you need to insert numbers or more than two pieces. The modern Python answer is the f-string — prefix a string with f and put expressions in curly braces:

name = "Ada"
balance = 1287.45
print(f"Hi {name}, your balance is ${balance:.2f}.")
# "Hi Ada, your balance is $1287.45."

The format spec after the colon controls how the value is displayed:

f"{1287.5:.2f}"          # "1287.50"   — 2 decimal places
f"{1287:,}"              # "1,287"     — thousands separator
f"{42:>6}"               # "    42"    — right-align in 6 chars
f"{42:0>4}"              # "0042"      — pad with zeros
f"{0.873:.1%}"           # "87.3%"     — percent

The older way (.format() and % formatting) still works but you should default to f-strings. They read top-to-bottom, they handle expressions, and they’re faster.

Section · 03

Indexing and slicing — same as lists

A string is technically a sequence of characters, which means you can index into it like a list. Indexes start at zero:

name = "Lovelace"
name[0]      # "L"
name[1]      # "o"
name[-1]     # "e"   — last
name[0:4]    # "Love"
name[-4:]    # "lace"
len(name)    # 8

One important thing: strings are immutable. You cannot change a character in place:

name = "lovelace"
name[0] = "L"        # TypeError — 'str' object does not support item assignment

# Build a new string instead:
name = "L" + name[1:]    # "Lovelace"

Immutability sounds annoying but it’s the reason strings can be used as dictionary keys, can be safely passed between functions without surprises, and are blazingly fast for comparison.

Section · 04

The 8 methods you'll actually use

Strings have ~40 methods. You’ll use the same eight 95% of the time. Memorize these:

text = "  Hello, World!  "

text.strip()              # "Hello, World!"  — trims whitespace from both ends
text.lower()              # "  hello, world!  "
text.upper()              # "  HELLO, WORLD!  "
text.replace("World", "York")   # "  Hello, York!  "

"Hello, World!".split(",")      # ["Hello", " World!"]
",".join(["a", "b", "c"])       # "a,b,c"

"hello.txt".startswith("hello") # True
"hello.txt".endswith(".txt")    # True

Worth knowing: every method returns a new string. The original is unchanged (because strings are immutable). This trips people up:

# WRONG — .strip() returns a new string; the original is unchanged
name = "  ada  "
name.strip()
print(name)        # "  ada  "   — still has spaces!

# RIGHT — capture the return value
name = name.strip()
print(name)        # "ada"

Real-world example: clean a user signup

raw_email = input("Email: ").strip().lower()

if not raw_email.endswith("@yorksims.com"):
    print("Sorry — yorksims.com emails only.")
elif "+" in raw_email:
    print("No plus-aliases please.")
else:
    username = raw_email.split("@")[0]
    print(f"Welcome, {username}.")

Six lines, four string methods, no regular expressions. That covers a huge percentage of real text-handling work. Reach for fancier tools (regex, parsers) only when these stop being enough.