Both methods are used to locate the position of a substring within a string but the major difference between find() and index() methods in Python is how they handle cases when the substring is not found. The find() method returns -1 in such cases, while index() method raises a ValueError.

Example of find()

s = "hello world"

# Finds the position of the substring "world"
print(s.find("world"))

# Returns -1 when substring is not found
print(s.find("python"))

# Finds the position of "o" starting from index 5
print(s.find("o", 5))

6
-1
7

Explanation:

• s.find("world") returns 6 because the substring "world" starts at index 6.
• s.find("python") returns -1 as substring "python" is not present in string.
• s.find("o", 5) starts the search from index 5 and finds the first occurrence of "o" at index 7.

Example of index()

s = "hello world"

# Finds position of the substring "world"
print(s.index("world"))

# Raises a ValueError when substring is not found
print(s.index("python"))

# Finds position of "o" starting from index 5
print(s.index("o", 5))

6

ERROR!
Traceback (most recent call last):
  File "<main.py>", line 7, in <module>
ValueError: substring not found

Explanation:

• s.index("world") returns 6 because substring "world" starts at index 6.
• When s.index("python") is executed then it raises a ValueError as the substring "python" is not in string.
• s.index("o", 5) starts searching from index 5 and finds the first occurrence of "o" at index 7.

When to use find() and index()?

• Use find() when we need to check if a substring exists and do not want to handle exceptions.
• Use index() when we are sure that the substring exists or we want an exception to be raised if it doesn't exist.