Python Strings

(toc)

A string in Python is a sequence of characters enclosed within single quotes ('), double quotes ("), or triple quotes (''' or """). Strings are one of the most commonly used data types in Python and are essential for working with text-based data.

Creating Strings

Strings can be created using:

• Single quotes: 'Hello'
• Double quotes: "World"
• Triple quotes: '''Multiline String''' or """Multiline String"""

Examples:

# Single and double-quoted strings
name = 'Alice'
greeting = "Hello, World!"

# Triple-quoted string for multi-line text
message = """This is a 
multi-line string."""

print(name)      # Output: Alice
print(greeting)  # Output: Hello, World!
print(message)   # Output: This is a multi-line string.

String Indexing and Slicing

Strings are indexed, allowing you to access specific characters or slices of the string.

Indexing

Python uses zero-based indexing.

text = "Python"

# Access individual characters
print(text[0])   # Output: P
print(text[-1])  # Output: n (negative indexing starts from the end)

Slicing

You can extract substrings using slicing syntax: string[start:end:step].

text = "Python Programming"

# Slice examples
print(text[0:6])    # Output: Python
print(text[7:])     # Output: Programming
print(text[:6])     # Output: Python
print(text[::2])    # Output: Pto rgamn (every second character)

String Immutability

Strings in Python are immutable, meaning they cannot be modified after creation.

Example:

text = "Hello"

# This will raise an error
# text[0] = 'h'  # TypeError: 'str' object does not support item assignment

However, you can create a new string based on the original.

text = "Hello"
text = "h" + text[1:]  # New string
print(text)  # Output: hello

String Operations

1. Concatenation

Combine strings using the + operator.

first_name = "John"
last_name = "Doe"
full_name = first_name + " " + last_name
print(full_name)  # Output: John Doe

2. Repetition

Repeat a string using the * operator.

text = "Hi! "
print(text * 3)  # Output: Hi! Hi! Hi! 

3. Membership Testing

Check if a substring exists using the in keyword.

text = "Python Programming"
print("Python" in text)  # Output: True
print("Java" in text)    # Output: False

String Methods

Python provides several built-in methods to manipulate strings.

Examples:

text = "  Python Programming  "

# Convert case
print(text.lower())     # Output: "  python programming  "
print(text.upper())     # Output: "  PYTHON PROGRAMMING  "

# Strip whitespace
print(text.strip())     # Output: "Python Programming"

# Replace substrings
print(text.replace("Python", "Java"))  # Output: "  Java Programming  "

# Split and join
words = text.strip().split()           # Split into a list
print(words)                           # Output: ['Python', 'Programming']
joined = "-".join(words)               # Join with a hyphen
print(joined)                          # Output: Python-Programming

String Formatting

Python provides several ways to format strings for dynamic content.

1. Using f-strings (formatted string literals)

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

2. Using format()

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

3. Using % Operator

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

Escape Characters

Use a backslash () to include special characters in strings.

Example:

text = "He said, "Python is amazing!"nLet's learn it together."
print(text)

Output:

He said, "Python is amazing!"
Let's learn it together.

String Iteration

Strings can be iterated over using a for loop.

Example:

text = "Python"
for char in text:
    print(char)

Output:

P
y
t
h
o
n