Strings

String Basics

Python strings (str) are immutable sequences of Unicode code points. Single and double quotes are equivalent; triple quotes (''' or """) allow multi-line literals.

name = "Alice"
bio = '''Multi-line
doc string.'''

# Concatenation, repetition, indexing
greeting = "hello" + " " + name
line = "-" * 20
first = name[0]              # 'A'
last = name[-1]               # 'e'

# Slicing — start:stop:step
print(name[1:4])           # 'lic'
print(name[::-1])          # 'ecilA' — reverse trick

# Membership and iteration
if "li" in name:
    ...
for ch in name:
    print(ch)

f-strings (PEP 498)

Formatted string literals — the modern way to interpolate. Embed any expression inside {}.

name = "Alice"
age = 30
price = 1234.567

print(f"{name} is {age}")
print(f"{name.upper()} — {age * 12} months")

# Format specs after a colon
print(f"{price:.2f}")          # '1234.57'
print(f"{price:,.2f}")         # '1,234.57'
print(f"{42:_>10}")            # '________42' — right-align, fill with _
print(f"{255:#x}")              # '0xff'

# Debug syntax (Python 3.8+)
print(f"{name=}, {age=}")        # name='Alice', age=30

Format Specifiers Quick Reference

Essential String Methods

s = "  Hello, World!  "

s.strip()              # 'Hello, World!' — removes leading/trailing whitespace
s.lower()              # '  hello, world!  '
s.upper()              # '  HELLO, WORLD!  '
s.title()              # '  Hello, World!  '

# Search
s.find("World")        # 9 (index) or -1 if not found
s.index("World")       # like find but raises ValueError
s.count("l")           # 3
"World" in s          # True
s.startswith("  H")    # True
s.endswith((".", "!", "  "))   # True — accepts tuple of options

# Replace and split
s.replace("World", "Python")
"a,b,c".split(",")       # ['a', 'b', 'c']
"a b\nc".split()         # ['a', 'b', 'c'] — whitespace split
",".join(["a", "b", "c"])     # 'a,b,c'

# Predicates (return bool)
"abc123".isalnum()       # True
"123".isdigit()          # True
"hello".isascii()        # True

Unicode and Encoding

Python 3 strings are Unicode by default. Bytes are a separate type for binary data.

s = "café"
len(s)                       # 4 — code points

# Encode str → bytes (for files, network, etc.)
data = s.encode("utf-8")        # b'caf\xc3\xa9' — 5 bytes
len(data)                    # 5

# Decode bytes → str
data.decode("utf-8")         # 'café'

# Get the code point of a character
ord("é")                  # 233
chr(233)                   # 'é'

Regular Expressions: the re module

import re

# Search anywhere in the string
m = re.search(r"(\d+)-(\d+)", "order 1234-5678 received")
if m:
    print(m.group(0))          # '1234-5678' (whole match)
    print(m.group(1))          # '1234' (first group)
    print(m.groups())          # ('1234', '5678')

# Match must start at beginning
re.match(r"hello", "hello world")

# Find all matches
re.findall(r"\w+@\w+\.\w+", text)

# Substitute
re.sub(r"\s+", " ", "too    much    space")
# 'too much space'

# Compile when reusing
email_re = re.compile(r"[\w.+-]+@[\w.-]+\.\w+")
for line in lines:
    for email in email_re.findall(line):
        ...