Python index() Method

What You’ll Learn

The index() method searches a string for a substring and returns the index of the first match. Unlike find(), it raises ValueError when nothing is found—so it is ideal when a missing substring means something went wrong. It is a core tool for locating text and slicing strings with confidence.

Definition and Usage

In Python, index() scans a string from left to right looking for a substring. When it finds a match, it returns the starting index (counting from 0). For example, in "Hello, World!", the substring "World" starts at index 7. If the substring does not appear, index() raises ValueError with the message substring not found.

📝 Syntax

The index() method accepts one required and two optional parameters:

string.index(sub, start, end)

Syntax Rules

• sub — the substring to search for (required).
• start — optional start index; search begins here (default 0).
• end — optional end index; search stops before this position.
• Return value — integer index of the first match on success.
• On failure — raises ValueError if sub is not found in the search range.
• Read-only — the original string is never modified.

↩ Return Value

On success, index() returns a non-negative integer: the index where the first character of sub appears in the string (or within the start–end slice). On failure, it raises ValueError—it never returns -1 like find().

text = "Hello, World!"

# Success: returns 7
position = text.index("World")

# Failure: raises ValueError
# text.index("Python")

Examples Gallery

Run these examples in any Python 3 interpreter. Indices start at 0 unless noted.

text = "Hello, World!"
position = text.index("World")

print("Text:    ", text)
print("Index:   ", position)
print("Char:    ", text[position])
print("Matched: ", text[position:position + 5])

Text:     Hello, World!
Index:    7
Char:     W
Matched:  World

text = "Hello, World!"

try:
    position = text.index("Python")
    print("Found at index:", position)
except ValueError as e:
    print("Could not find 'Python':", e)

Could not find 'Python': substring not found

text = "Hello, Hello, World!"

first = text.index("Hello")
second = text.index("Hello", 7, 15)

print("First Hello at: ", first)
print("Second Hello at:", second)

First Hello at:  0
Second Hello at: 7

text = "Hello, World!"

try:
    text.index("world")
except ValueError:
    print("Lowercase 'world' not found (case-sensitive).")

position = text.index("World")
print("Exact match 'World' at index:", position)

Lowercase 'world' not found (case-sensitive).
Exact match 'World' at index: 7

text = "Hello, World!"
sub = "Python"

print("find(): ", text.find(sub))

try:
    print("index():", text.index(sub))
except ValueError as e:
    print("index() raised ValueError:", e)

find():  -1
index() raised ValueError: substring not found

🚀 Common Use Cases

• Strict parsing — require a delimiter like "@" in email addresses and fail fast if it is missing.
• Config validation — ensure required keys or markers appear in a template string.
• Protocol messages — locate fixed headers in known-format log lines.
• Text extraction — find a delimiter, then slice from that index onward.
• Asserting invariants — use index() in tests or internal code where absence should never happen.

📝 Notes

• index() returns only the first match; use a loop with an increasing start to find later occurrences.
• For the last occurrence, use rindex() instead.
• Like find(), index() is case-sensitive—normalize case when needed.
• An empty substring "" always matches at index 0 (or at start when given).
• Store the result in a variable if you need the index more than once to avoid redundant searches.

💡 Best Practices