Section 20.7. Reversing Sequences

20.7. Reversing Sequences

Reversal 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\

 def reverse(list):               # recursive     if list == []:         return []     else:         return reverse(list[1:]) + list[:1] def ireverse(list):              # iterative     res = []     for x in list: res = [x] + res     return res 

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:

  • reverse uses the not operator to detect the end of the sequence and returns the empty sequence itself, rather than an empty list constant. Since the empty sequence is the type of the original argument, the + operation always builds the correct type sequence as the recursion unfolds.

  • ireverse makes use of the fact that slicing a sequence returns a sequence of the same type. It first initializes the result to the slice [:0], a new, empty slice of the argument's type. Later, it uses slicing to extract one-node sequences to add to the result's front, instead of a list constant.

Example 20-22. PP3E\Dstruct\Classics\

 def reverse(list):     if not list:                               # empty? (not always [])         return list                            # the same sequence type     else:         return reverse(list[1:]) + list[:1]    # add front item on the end def ireverse(list):     res = list[:0]                             # empty, of same type     for i in range(len(list)):         res = list[i:i+1] + res                # add each item to front     return res 

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)) 

Programming Python
Programming Python
ISBN: 0596009259
EAN: 2147483647
Year: 2004
Pages: 270
Authors: Mark Lutz

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