index and find

[Shi Mu]

what is the difference between index and find in the module of string?
for both "find" and "index", I got the position of the letter.

 

[Sam Pointon]

s = 'foobar'
Traceback (most recent call last):
File "<pyshell#32>", line 1, in -toplevel-
s.index('z')
ValueError: substring not found


Pretty self explanatory.