Strings are sequences of characters, used to represent text in programming. In Python, strings are immutable, meaning once a string is created, its contents cannot be changed. Any operation that appears to modify a string actually creates a new string.

Creating Strings

You can create strings by enclosing characters in single quotes ('), double quotes ("), or triple quotes (''' or """). Triple quotes are useful for multi-line strings or docstrings.

python
# Single quotes
str1 = 'Hello, Python!'

# Double quotes
str2 = "Learning Python is fun."

# Triple quotes for multi-line strings
str3 = """This is a
multi-line string.
It preserves newlines."""

# Triple quotes also for single-line, but less common
str4 = '''Another single-line string'''

print(str1)
print(str2)
print(str3)
print(type(str1)) # Output: <class 'str'>

String Concatenation

You can combine (concatenate) strings using the + operator. This creates a new string.

python
greeting = "Hello"
name = "Alice"
full_greeting = greeting + ", " + name + "!"
print(full_greeting) # Output: Hello, Alice!

# You can also use the * operator to repeat a string
repeated_string = "abc" * 3
print(repeated_string) # Output: abcabcabc

String Formatting

Combining strings with variables is a common task.

Python offers several ways to format strings:

1. f-strings (Formatted String Literals - Python 3.6+): This is the most modern and recommended way. Prefix the string with f or F and place variable names directly inside curly braces {}.

   python
   item = "book"
   price = 19.99
   message_f = f"The {item} costs ${price:.2f}."
   print(message_f) # Output: The book costs $19.99.

2. str.format() method: Older but still widely used. Uses curly braces {} as placeholders.

   python
   message_format = "The {} costs ${:.2f}.".format(item, price)
   print(message_format) # Output: The book costs $19.99.

3. Old-style % formatting: Similar to C's printf. Less common in modern Python due to f-strings.

   python
   message_old = "The %s costs $%.2f." % (item, price)
   print(message_old) # Output: The book costs $19.99.

String Indexing and Slicing

Strings are ordered sequences, meaning each character has an index (position). Indexing starts at

0. • Indexing: Access a single character using square brackets [].

• Slicing: Extract a substring (a portion of the string) using [start:end:step].

python
my_string = "Python Programming"

# Accessing characters
print(my_string[0])   # Output: P (first character)
print(my_string[7])   # Output: P (character at index 7)
print(my_string[-1])  # Output: g (last character, negative indexing)
print(my_string[-12]) # Output: o (12th character from the end)

# Slicing (end index is exclusive)
print(my_string[0:6])   # Output: Python
print(my_string[9:])    # Output: rogramming (from index 9 to the end)
print(my_string[:6])    # Output: Python (from beginning to index 6)
print(my_string[7:18])  # Output: Programming
print(my_string[::2])   # Output: Pto rmai (every second character)
print(my_string[::-1])  # Output: gnimmargorP nohtyP (reverse string)

Common String Methods

Python strings come with many useful built-in methods (functions that belong to an object):

• len(string): Returns the length of the string.
• str.upper(): Returns a new string with all characters in uppercase.
• str.lower(): Returns a new string with all characters in lowercase.
• str.strip(): Returns a new string with leading and trailing whitespace removed.
• str.replace(old, new): Returns a new string with all occurrences of old replaced by new.
• str.split(delimiter): Returns a list of substrings split by the given delimiter.
• str.find(substring): Returns the lowest index of the substring if found, -1 otherwise.

python
sentence = "   Hello, Python World!   "

print(len(sentence))            # Output: 26
print(sentence.upper())         # Output:    HELLO, PYTHON WORLD!   
print(sentence.lower())         # Output:    hello, python world!   
print(sentence.strip())         # Output: Hello, Python World!
print(sentence.replace("Python", "Java")) # Output:    Hello, Java World!   
print("apple,banana,cherry".split(',')) # Output: ['apple', 'banana', 'cherry']
print("Python".find("tho"))      # Output: 2
print("Python".find("xyz"))      # Output: -1

Strings are fundamental for handling text data, which is ubiquitous in programming. Mastering their creation, manipulation, and formatting is a crucial skill.

Key Takeaways:

• Strings are immutable sequences of characters, enclosed in quotes.
• Concatenate with + and repeat with *.
• Use f-strings for modern and efficient string formatting.
• Access individual characters via indexing ([index]) and substrings via slicing ([start:end]).
• Utilize built-in string methods like len(), upper(), lower(), strip(), replace(), and split() for common manipulations.