Suppose we write the following function:

>>> def fetcher(obj, index):
...     return obj[index]
...

There’s not much to this function—it simply indexes an object on a passed-in index. In normal operation, it returns the result of a legal index:

>>> x = 'spam'
>>> fetcher(x, 3)                         # Like x[3]
'm'

However, if we ask this function to index off the end of the string, an exception will be triggered when the function tries to run obj[index]. Python detects out-of-bounds indexing for sequences and reports it by raising (triggering) the built-in IndexError exception: