Python — Strings

Creating strings

Python strings are delimited by single or double quotes — they're identical:

name = "Ada"
greeting = 'Hello'

Triple quotes handle multi-line strings:

poem = """
Roses are red,
Violets are blue,
Python is awesome,
And so are you.
"""

String indexing

Access individual characters by position (zero-indexed):

word = "Python"
print(word[0])   # P
print(word[5])   # n
print(word[-1])  # n (last character)
print(word[-2])  # o (second to last)

Negative indices count from the end — -1 is the last character.

Slicing

Extract substrings with the [start:stop:step] syntax:

text = "Hello, World!"

print(text[0:5])    # Hello (positions 0-4)
print(text[7:12])   # World
print(text[:5])     # Hello (start from beginning)
print(text[7:])     # World! (go to end)
print(text[::2])    "HloWrd (every other character)
print(text[::-1])   # !dlroW ,olleH (reversed)

Slicing never raises an index error — Python handles out-of-range gracefully.

String methods

text = "  Hello, World!  "

# Case methods
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!  "

# Searching
print(text.find("World"))   # 9 (index of first occurrence)
print(text.find("Python"))  # -1 (not found)
print(text.count("l"))      # 3
print("hello".startswith("he"))  # True
print("hello".endswith("lo"))    # True

# Trimming
print(text.strip())    # "Hello, World!" — removes whitespace
print(text.lstrip())   # "Hello, World!  " — left only
print(text.rstrip())   # "  Hello, World!" — right only

# Replacing and splitting
print(text.replace("World", "Python"))  # "  Hello, Python!  "
words = text.strip().split(", ")        # ["Hello", "World!"]

String concatenation

first = "Hello"
second = "World"
combined = first + " " + second  # "Hello World"

# Repetition
line = "-" * 30  # "------------------------------"

For building strings in loops, join() is more efficient:

words = ["Python", "is", "awesome"]
sentence = " ".join(words)  # "Python is awesome"
csv = ",".join(words)       # "Python,is,awesome"

f-strings — the modern way

f-strings embed expressions directly:

name = "Ada"
age = 36
score = 95.678

# Basic interpolation
print(f"Hello, {name}!")

# Expressions
print(f"Next year you'll be {age + 1}.")

# Format specifiers
print(f"Score: {score:.1f}")     # 95.7
print(f"Score: {score:.0f}")     # 96
print(f"Number: {42:05d}")       # 00042
print(f"Price: ${19.99:.2f}")    # $19.99
print(f"Percent: {0.856:.1%}")   # 85.6%
print(f"Right-aligned: {42:>10}") #         42
print(f"Centered: {'hi':^10}")   #     hi

String methods for validation

text = "Hello123"

print(text.isalpha())     # False — contains digits
print(text.isalnum())     # True  — letters and digits
print(text.isdigit())     # False — contains letters
print("12345".isdigit())  # True  — all digits
print("hello".islower())  # True
print("HELLO".isupper())  # True
print("Hello".istitle())  # True — title case
print("   ".isspace())    # True  — all whitespace

Raw strings

Prefix with r to treat backslashes as literal characters:

path = r"C:\Users\Ada\Documents"
print(path)  # C:\Users\Ada\Documents

regex = r"\d+"
print(regex)  # \d+ (not a digit pattern yet — just a string)

Raw strings are essential for Windows file paths and regular expressions.

String encoding

Python strings are Unicode by default:

text = "Hello, 世界!"
print(len(text))  # 10 — characters, not bytes

# Encode to bytes
encoded = text.encode("utf-8")
print(type(encoded))  # <class 'bytes'>
print(len(encoded))   # 14 — bytes needed

# Decode back to string
decoded = encoded.decode("utf-8")
print(decoded)  # Hello, 世界!

Common string patterns

Checking substrings

email = "user@example.com"
if "@" in email and "." in email:
    print("Looks like an email")

Formatting names

first = "Ada"
last = "Lovelace"
full = f"{first} {last}"

Building paths

import os
folder = "documents"
file = "report.pdf"
path = os.path.join(folder, file)

Strings are immutable

s = "Hello"
# s[0] = "h"  # TypeError: 'str' object does not support item assignment

# Workaround — create a new string
s = "h" + s[1:]  # "hello"

Every string method returns a new string. The original stays unchanged.

Mini Practice

1. Reverse a string using slicing: "Hello" → "olleH"
2. Count how many times a letter appears in a sentence
3. Use f-strings to format a receipt with item names and prices
4. Check if a string is a palindrome
5. Split a sentence into words and rejoin them in reverse order

Next: operators — math and comparisons →