20.7. Reversing SequencesReversal of collections is another typical operation. We can code it either recursively or iteratively in Python, and as functions or class methods. Example 20-21 is a first attempt at two simple reversal functions. Example 20-21. PP3E\Dstruct\Classics\rev1.py
Both reversal functions work correctly on lists. But if we try reversing nonlist sequences (strings, tuples, and so on) we're in trouble: the ireverse function always returns a list for the result regardless of the type of sequence passed: >>> ireverse("spam") ['m', 'a', 'p', 's'] Much worse, the recursive reverse version won't work at all for nonlistsit gets stuck in an infinite loop. The reason is subtle: when reverse reaches the empty string (""), it's not equal to the empty list ([]), so the else clause is selected. But slicing an empty sequence returns another empty sequence (indexes are scaled): the else clause recurs again with an empty sequence, without raising an exception. The net effect is that this function gets stuck in a loop, calling itself over and over again until Python runs out of memory. The versions in Example 20-22 fix both problems by using generic sequence handling techniques:
Example 20-22. PP3E\Dstruct\Classics\rev2.py
These functions work on any sequence, and they return a new sequence of the same type as the sequence passed in. If we pass in a string, we get a new string as the result. In fact, they reverse any sequence object that responds to slicing, concatenation, and leneven instances of Python classes and C types. In other words, they can reverse any object that has sequence interface protocols. Here they are working on lists, strings, and tuples: % python >>> from rev2 import * >>> reverse([1,2,3]), ireverse([1,2,3]) ([3, 2, 1], [3, 2, 1]) >>> reverse("spam"), ireverse("spam") ('maps', 'maps') >>> reverse((1.2, 2.3, 3.4)), ireverse((1.2, 2.3, 3.4)) ((3.4, 2.3, 1.2), (3.4, 2.3, 1.2)) |