Strings and Text Manipulation¶

A string in Python is a sequence of Unicode characters enclosed in single quotes ('...') or double quotes ("..."). Strings are immutable, meaning they cannot be modified after creation.

Defining Strings¶

# Single quotes and double quotes behave identically
str1 = 'Python'
str2 = "Python"

# Escaping special characters with backslash (\)
quote = 'He said, "It\'s raining."'

# Multiline strings using triple quotes
multiline = """Line 1
Line 2
Line 3"""

# Raw strings (prefix r or R) ignore backslash escape sequences
raw_path = r"C:\new_folder\test.txt"

String Indexing¶

Python sequences use zero-based indexing. You can access characters from the beginning using non-negative integers ($0, 1, 2, ...$) or from the end using negative integers ($-1, -2, -3, ...$).

Consider the string "PYTHON":

word = "PYTHON"

print(word[0])   # First character: 'P'
print(word[3])   # Fourth character: 'H'
print(word[-1])  # Last character: 'N'
print(word[-2])  # Second to last: 'O'

String Slicing¶

Slicing allows you to obtain a substring from a string.

Syntax¶

string[start:stop:step]

• start: Inclusive beginning index (defaults to 0).
• stop: Exclusive ending index (slice stops before this index).
• step: Stride or step count between characters (defaults to 1).

s = "ProjectPy"

print(s[0:7])   # 'Project' (from index 0 to index 6)
print(s[:7])    # 'Project' (start defaults to 0)
print(s[7:])    # 'Py'      (slice from index 7 to end)
print(s[-2:])   # 'Py'      (last 2 characters)
print(s[::2])   # 'PoetP'   (every 2nd character)
print(s[::-1])  # 'yPtcejorP' (reverse string)

Strings are Immutable¶

Attempting to change an individual character by index raises a TypeError:

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

# Correct approach: Create a new string
new_word = "J" + word[1:]
print(new_word)  # 'Jython'

Formatted String Literals (f-strings)¶

Formatted string literals (f-strings) provide a concise way to embed expressions inside string literals:

name = "Ada"
score = 98.456

# Variable interpolation
print(f"Student: {name}")

# Number formatting
print(f"Final Score: {score:.2f}")

# Inline expressions
print(f"Double Score: {score * 2:.1f}")

Common String Methods¶