str.index() Function in Python

The str.index() function in Python is similar to str.find(), as it is used to search for a specified substring within a string and return the index of its first occurrence. However, there is a key difference: if the substring is not found, str.index() raises a ValueError instead of returning -1. This makes str.index() useful when you expect the substring to be present and want to handle cases where it is not found with an exception.

Syntax:

string.index(substring, start, end)

• substring: The substring you want to search for.
• start (optional): The starting index from where the search should begin. The default is 0.
• end (optional): The ending index where the search should stop. The default is the end of the string.

Example 1: Basic usage

# Example 1: Finding a substring
my_string = "Hello, world!"
index = my_string.index("world")
print(index)  # Output: 7

In this example:

• The substring "world" is found at index 7 in the string "Hello, world!".

Example 2: Substring not found

# Example 2: Substring not found
my_string = "Hello, world!"
try:
    index = my_string.index("Python")
except ValueError as e:
    print(e)  # Output: substring not found

Here:

• The substring "Python" is not found in the original string, causing str.index() to raise a ValueError, which is caught and printed.

Example 3: Specifying start and end indices

# Example 3: Specifying start and end indices
my_string = "Hello, world! Welcome to the world of Python."
index = my_string.index("world", 10)
print(index)  # Output: 27

In this case:

• The search for "world" starts from index 10, and the first occurrence of "world" after that is found at index 27.

Example 4: Using index() with overlapping substrings

# Example 4: Overlapping substrings
my_string = "banana"
index = my_string.index("ana")
print(index)  # Output: 1

Here:

• The substring "ana" is found starting at index 1.

Key Points:

• str.index() returns the index of the first occurrence of the specified substring.
• It is case-sensitive, meaning "Hello" and "hello" are considered different.
• If the substring is not found, it raises a ValueError.
• You can specify optional start and end parameters to limit the search range within the string.
• The function does not modify the original string.

Example 5: Case sensitivity

# Example 5: Case sensitivity
my_string = "Hello, world!"
try:
    index = my_string.index("hello")
except ValueError as e:
    print(e)  # Output: substring not found

In this example:

• The search for "hello" (lowercase) raises a ValueError because it is not found in the original string.

Example 6: Multiple occurrences

# Example 6: Multiple occurrences
my_string = "apple banana apple orange"
index = my_string.index("apple")
print(index)  # Output: 0

Here:

• The first occurrence of "apple" is found at index 0, even though there is a second occurrence later in the string.