Strings in Python

1. Introduction to Strings

A string is an ordered sequence of characters (letters, digits, symbols, spaces) enclosed within single quotes ('), double quotes ("), or triple quotes (""" or ''').

In Python, strings are objects of the built-in str class and are immutable (cannot be modified after creation).

2. Creating Strings

# Single quotes
s1 = 'Hello'

# Double quotes
s2 = "Python"

# Triple quotes (multi-line strings)
s3 = """This is a
multi-line
string."""

# Empty string
empty = ""

# String from input
name = input("Enter name: ")

3. String Indexing

Each character in a string has a position called an index. Indexing starts from 0.

text = "Python"
# Index:  0 1 2 3 4 5
# Characters: P y t h o n

print(text[0])   # P
print(text[2])   # t
print(text[-1])  # n (negative index: last character)
print(text[-3])  # h (third from end)

Important: Attempting to access an index outside the range raises IndexError.

4. String Slicing

Slicing extracts a portion of a string. Syntax: string[start:stop:step]

text = "Python Programming"
print(text[0:6])     # Python (index 0 to 5)
print(text[7:18])    # Programming
print(text[:6])      # Python (start defaults to 0)
print(text[7:])      # Programming (stop defaults to end)
print(text[::2])     # Pto rgamn (every 2nd char)
print(text[::-1])    # gnimmargorP nohtyP (reverse)
print(text[-11:-4])  # Progra (negative indices)

5. Strings are Immutable

Strings cannot be changed after creation. Any operation that modifies a string creates a new string.

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

# Correct way: create a new string
text = "J" + text[1:]  # "Jello"

6. String Operators

7. Traversing a String

# Using for loop (by character)
for char in "Python":
    print(char)

# Using for loop with index
text = "Python"
for i in range(len(text)):
    print(f"Index {i}: {text[i]}")

8. String Methods

9. String Formatting

# f-strings (Python 3.6+)
name = "Riya"
age = 17
print(f"My name is {name} and I am {age} years old.")

# format() method
print("My name is {} and I am {} years old.".format(name, age))

# % formatting
print("My name is %s and I am %d years old." % (name, age))

10. Unicode and ASCII

Python strings support Unicode, allowing characters from all languages.

# ord() - returns Unicode code point
print(ord("A"))   # 65
print(ord("a"))   # 97
print(ord("0"))   # 48

# chr() - returns character from code point
print(chr(65))    # A
print(chr(97))    # a

# Unicode characters
print("\u0041")   # A
print("\U0001F600")  # smiley emoji

11. Escape Sequences

print("Line1\nLine2")   # Two lines
print("Column1\tColumn2")  # Tab-separated