str.index() Find (raises if missing)
Last updated: 2026-09-22
str.index() Find (raises if missing)
str.index(sub) is like find(), but raises ValueError when not found.
| Category | String Methods |
|---|---|
| Kind | Built-in Type Method |
| Python Version | all |
📝 Syntax
str.index(sub[, start[, end]])⚙️ Parameters
| sub Required | The substring to search for. |
|---|---|
| start Optional | Start index of the range (inclusive); None means the beginning.（Default：None） |
| end Optional | End index of the range (exclusive); None means the end.（Default：None） |
Returns：int. The lowest index where sub is found; raises ValueError if absent.
Mutates the original：No (returns a new object)
💥 Raises
ValueError— Raised when sub is not found in the range.
▶ Example
Edit the code, press Run, and see the result in the output panel:
Output：
2
not found: substring not found💡 Related Cases
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❓ FAQ
QWhat is the difference between index() and find()?
ATheir behavior is almost identical, with one difference: when not found, find() returns -1 while index() raises ValueError: substring not found.
QWhen should I use index()?
AUse index() when the substring "must exist" and not finding it is a logic error, so the exception exposes the problem immediately; otherwise find() is safer.
QHow do I avoid index() raising an exception?
ACheck with in first: if 'll' in s: print(s.index('ll')); or catch ValueError with try/except.
