search

`str.index`

Like find(), but raises ValueError if the substring is not found. Use when the substring is expected to exist and its absence is an error.

Syntax

str.index(sub[, start[, end]])

Parameters

Parameter	Purpose
sub	substring to locate
start	optional slice start
end	optional slice end

Examples

>>> 'hello world'.index('world')
6

returns index of first match

>>> 'hello'.index('z')
Traceback (most recent call last):
  ...
ValueError: substring not found

raises ValueError on miss (unlike find)

>>> 'ababab'.index('a', 2)
2

start argument narrows search window

>>> 'abc'.index('')
0

empty substring matches at 0

Gotcha

Use find() if you want a sentinel -1 instead of an exception. Prefer index() when a missing substring genuinely indicates a bug.

Related methods

Returns the lowest index in the string where substring sub is found within the slice [start:end]. Returns -1 if the substring is not found, unlike index() which raises ValueError.

Returns the highest index in the string where substring sub is found within [start:end]. Returns -1 if the substring is not found.

Returns the number of non-overlapping occurrences of substring sub in [start:end]. Overlapping matches are not counted.

Returns True if the string starts with the given prefix, otherwise False. prefix may also be a tuple of strings to test against multiple candidates.

Returns True if the string ends with the given suffix, otherwise False. suffix may be a tuple of strings to test multiple candidates at once.

← All Python string methods · Python built-ins