Strings in Python

Strings are sequences of characters used to represent text data.

String Creation

# Single quotes
name = 'Alice'

# Double quotes
greeting = "Hello, World!"

# Triple quotes for multi-line strings
poem = """Roses are red,
Violets are blue,
Python is great,
And so are you!"""

print(name)
print(greeting)
print(poem)

Escape Sequences

# Common escape sequences
text = "Hello\nWorld"        # Newline
text = "Hello\tWorld"        # Tab
text = "He said \"Hello\""    # Double quote
text = 'It\'s raining'        # Single quote
text = "Path: C:\\folder"     # Backslash

Raw Strings (ignore escape sequences)

# Raw string
path = r"C:\Users\Documents\file.txt"
print(path)  # C:\Users\Documents\file.txt

String Indexing

Strings are indexed, starting from 0.

text = "Python"

print(text[0])   # P
print(text[1])   # y
print(text[-1])  # n (last character)
print(text[-2])  # o (second to last)

String Slicing

Extract substrings using slicing. Slicing uses the syntax string[start:end:step].

Basic slicing syntax:

text = "Hello, World!"

# [start:end] - extract from start to end-1
print(text[0:5])    # Hello (characters at indices 0,1,2,3,4)
print(text[7:12])   # World (characters at indices 7,8,9,10,11)

# [:end] - from beginning to end-1
print(text[:5])     # Hello (same as text[0:5])

# [start:] - from start to end of string
print(text[7:])     # World! (from index 7 to end)

# [:] - entire string
print(text[:])      # Hello, World! (copy of whole string)

Step parameter for advanced slicing:

text = "Hello, World!"

# [::step] - every step-th character
print(text[::2])    # Hlo ol! (every 2nd character)
print(text[1::2])   # el,Wrd (every 2nd character starting from index 1)

# Negative step - reverse direction
print(text[::-1])   # !dlroW ,olleH (reverse the string)
print(text[::-2])   # !lo le (reverse, every 2nd character)

Negative indices:

text = "Python"

print(text[-1])     # n (last character)
print(text[-2])     # o (second to last)
print(text[-3:])    # hon (last 3 characters)
print(text[:-2])    # Pyth (all except last 2)

Practical slicing examples:

# Extract domain from email
email = "user@example.com"
domain = email.split("@")[1]
print(domain)  # example.com

# Or using slicing (if you know the position)
at_index = email.find("@")
if at_index != -1:
    domain = email[at_index + 1:]
    print(domain)  # example.com

# Remove file extension
filename = "document.txt"
name_without_ext = filename[:-4]  # Remove last 4 characters (.txt)
print(name_without_ext)  # document

# Get first and last n characters
text = "Hello, World!"
first_3 = text[:3]      # Hel
last_3 = text[-3:]      # ld!
print(f"First 3: {first_3}, Last 3: {last_3}")

# Skip characters from both ends
text = "xxxHello, World!xxx"
cleaned = text[3:-3]    # Remove 3 chars from start and end
print(cleaned)          # Hello, World!

String slicing vs string methods:

text = "  Hello, World!  "

# Using slicing
trimmed = text[2:-2]    # Manual trimming
print(repr(trimmed))    # 'Hello, World!'

# Using string methods (better)
trimmed = text.strip()
print(repr(trimmed))    # 'Hello, World!'

Slicing creates new strings (immutable behavior):

original = "Python"
sliced = original[1:4]  # "yth"

print(original)  # Python (unchanged)
print(sliced)    # yth (new string)

# Strings are immutable - this would error:
# original[0] = "J"  # TypeError!

String Immutability

Strings cannot be changed after creation.

text = "Hello"
# text[0] = "h"  # This will cause an error

# To "modify" a string, create a new one
new_text = "h" + text[1:]
print(new_text)  # hello

String Methods - Character Case

upper() - Convert to uppercase

text = "hello world"
print(text.upper())  # HELLO WORLD

lower() - Convert to lowercase

text = "HELLO WORLD"
print(text.lower())  # hello world

capitalize() - Capitalize first letter

text = "hello world"
print(text.capitalize())  # Hello world

title() - Capitalize first letter of each word

text = "hello world python"
print(text.title())  # Hello World Python

swapcase() - Swap case of all characters

text = "Hello World"
print(text.swapcase())  # hELLO wORLD

find() - Find the position of a substring

text = "Hello, World!"
print(text.find("World"))  # 7
print(text.find("Python")) # -1 (not found)

rfind() - Find from the right

text = "Hello, World, Hello!"
print(text.rfind("Hello"))  # 13

index() - Like find() but raises ValueError if not found

text = "Hello, World!"
print(text.index("World"))  # 7
# print(text.index("Python"))  # ValueError!

count() - Count occurrences of substring

text = "hello hello world"
print(text.count("hello"))  # 2

replace() - Replace occurrences of a substring

text = "Hello, World!"
new_text = text.replace("World", "Python")
print(new_text)  # Hello, Python!

# Replace multiple occurrences
text = "cat cat cat"
print(text.replace("cat", "dog"))  # dog dog dog

split() - Split string into a list

text = "apple,banana,cherry"
fruits = text.split(",")
print(fruits)  # ['apple', 'banana', 'cherry']

# Split by whitespace
text = "Hello World Python"
words = text.split()
print(words)  # ['Hello', 'World', 'Python']

# Limit splits
text = "a,b,c,d,e"
parts = text.split(",", 2)
print(parts)  # ['a', 'b', 'c,d,e']

join() - Join elements of a list into a string

fruits = ['apple', 'banana', 'cherry']
text = ", ".join(fruits)
print(text)  # apple, banana, cherry

# Join with different separator
text = "-".join(fruits)
print(text)  # apple-banana-cherry

Other Useful String Methods

text = "  Hello, World!  "

# strip() - Remove whitespace from both ends
print(text.strip())  # "Hello, World!"

# lstrip() - Remove whitespace from left
print(text.lstrip())  # "Hello, World!  "

# rstrip() - Remove whitespace from right
print(text.rstrip())  # "  Hello, World!"

text = "Python Programming"

# startswith() - Check if string starts with substring
print(text.startswith("Python"))  # True

# endswith() - Check if string ends with substring
print(text.endswith("ming"))  # True

# count() - Count occurrences of substring
print(text.count("m"))  # 2

# capitalize() - Capitalize first letter
print(text.capitalize())  # Python programming

# title() - Capitalize first letter of each word
print(text.title())  # Python Programming

# isalpha() - Check if all characters are alphabetic
print("Python".isalpha())  # True
print("Python3".isalpha())  # False

# isdigit() - Check if all characters are digits
print("123".isdigit())  # True
print("123abc".isdigit())  # False

# isalnum() - Check if all characters are alphanumeric
print("Python123".isalnum())  # True
print("Python 123".isalnum())  # False (space is not alphanumeric)

String Formatting

Using f-strings (Python 3.6+)

name = "Alice"
age = 25
print(f"My name is {name} and I am {age} years old.")

Using format() method

name = "Alice"
age = 25
print("My name is {} and I am {} years old.".format(name, age))

# With positional arguments
print("My name is {0} and I am {1} years old.".format(name, age))

# With keyword arguments
print("My name is {name} and I am {age} years old.".format(name=name, age=age))

Using % formatting (older style)

name = "Alice"
age = 25
print("My name is %s and I am %d years old." % (name, age))

String Operations and Tricks

String concatenation

# Using +
greeting = "Hello" + " " + "World"
print(greeting)  # Hello World

# Using join (more efficient for many strings)
words = ["Hello", "World"]
greeting = " ".join(words)
print(greeting)  # Hello World

String repetition

print("Ha" * 3)  # HaHaHa

String length

text = "Hello"
print(len(text))  # 5

Check if substring exists

text = "Hello, World!"
print("World" in text)  # True
print("Python" not in text)  # True

String comparison

print("apple" < "banana")  # True (lexicographical order)
print("Apple" < "apple")   # True (ASCII order)

Common String Use Cases

Parsing CSV data

csv_line = "Alice,25,Engineer"
name, age, profession = csv_line.split(",")
print(f"Name: {name}, Age: {age}, Profession: {profession}")

URL parsing

url = "https://www.example.com/path?param=value"
if url.startswith("https://"):
    print("Secure connection")
domain = url.split("/")[2]
print(f"Domain: {domain}")

Text processing

text = "This is a SAMPLE text with MIXED case."
# Normalize to title case
normalized = text.lower().capitalize()
print(normalized)

Password validation

password = "MyPass123"
has_upper = any(c.isupper() for c in password)
has_lower = any(c.islower() for c in password)
has_digit = any(c.isdigit() for c in password)
is_valid = len(password) >= 8 and has_upper and has_lower and has_digit
print(f"Password valid: {is_valid}")

String Performance Tips

• Use join() instead of + for concatenating many strings

• Use in operator for substring checking (more efficient than find())

• Prefer f-strings for formatting (fastest and most readable)

• Use string methods instead of manual loops when possible

String Encoding

# Strings are Unicode in Python 3
text = "Hello, 世界"
print(text.encode('utf-8'))  # b'Hello, \xe4\xb8\x96\xe7\x95\x8c'

# Decode bytes back to string
bytes_data = b'Hello, \xe4\xb8\x96\xe7\x95\x8c'
decoded = bytes_data.decode('utf-8')
print(decoded)  # Hello, 世界