Cooking with Python, Part 2

O'Reilly Book Excerpts: Python Cookbook, Second Edition

Cooking with Python, Part 2

Editor's note: If you missed recipes from part one of this two-part series of excerpts from Python Cookbook, 2nd Edition, then you missed how to handle international text with Unicode, and how to select elements from an unsorted sequence. For all 200-plus recipes, check out the latest edition.

Recipe 6.11: Implementing a Ring Buffer

Credit: Sébastien Keim, Paul Moore, Steve Alexander, Raymond Hettinger

Problem

You want to define a buffer with a fixed size, so that, when it fills up, adding another element overwrites the first (oldest) one. This kind of data structure is particularly useful for storing log and history information.

Solution

This recipe changes the buffer object's class on the fly, from a nonfull buffer class to a full buffer class, when the buffer fills up:

class RingBuffer(object):
    """ class that implements a not-yet-full buffer """
    def _ _init_ _(self, size_max):
        self.max = size_max
        self.data = [  ]
    class _ _Full(object):
        """ class that implements a full buffer """
        def append(self, x):
            """ Append an element overwriting the oldest one. """
            self.data[self.cur] = x
            self.cur = (self.cur+1) % self.max
        def tolist(self):
            """ return list of elements in correct order. """
            return self.data[self.cur:] + self.data[:self.cur]
    def append(self, x):
        """ append an element at the end of the buffer. """
        self.data.append(x)
        if len(self.data) == self.max:
            self.cur = 0
            # Permanently change self's class from non-full to full
            self._ _class_ _ = _ _Full
    def tolist(self):
        """ Return a list of elements from the oldest to the newest. """
        return self.data
# sample usage
if _ _name_ _ == '_ _main_ _':
    x = RingBuffer(5)
    x.append(1); x.append(2); x.append(3); x.append(4)
    print x._ _class_ _, x.tolist( )
    x.append(5)
    print x._ _class_ _, x.tolist( )
    x.append(6)
    print x.data, x.tolist( )
    x.append(7); x.append(8); x.append(9); x.append(10)
    print x.data, x.tolist( )

Discussion

A ring buffer is a buffer with a fixed size. When it fills up, adding another element overwrites the oldest one that was still being kept. It's particularly useful for the storage of log and history information. Python has no direct support for this kind of structure, but it's easy to construct one. The implementation in this recipe is optimized for element insertion.

The notable design choice in the implementation is that, since these objects undergo a nonreversible state transition at some point in their lifetimes—from nonfull buffer to full buffer (and behavior changes at that point)—I modeled that by changing self._ _class_ _. This works just as well for classic classes as for new-style ones, as long as the old and new classes of the object have the same slots (e.g., it works fine for two new-style classes that have no slots at all, such as RingBuffer and _ _Full in this recipe). Note that, differently from other languages, the fact that class _ _Full is implemented inside class RingBuffer does not imply any special relationship between these classes; that's a good thing, too, because no such relationship is necessary.

Changing the class of an instance may be strange in many languages, but it is an excellent Pythonic alternative to other ways of representing occasional, massive, irreversible, and discrete changes of state that vastly affect behavior, as in this recipe. Fortunately, Python supports it for all kinds of classes.

Ring buffers (i.e., bounded queues, and other names) are quite a useful idea, but the inefficiency of testing whether the ring is full, and if so, doing something different, is a nuisance. The nuisance is particularly undesirable in a language like Python, where there's no difficulty—other than the massive memory cost involved—in allowing the list to grow without bounds. So, ring buffers end up being underused in spite of their potential. The idea of assigning to _ _class_ _ to switch behaviors when the ring gets full is the key to this recipe's efficiency: such class switching is a one-off operation, so it doesn't make the steady-state cases any less efficient.

Alternatively, we might switch just two methods, rather than the whole class, of a ring buffer instance that becomes full:

class RingBuffer(object):
    def _ _init_ _(self,size_max):
        self.max = size_max
        self.data = [  ]
    def _full_append(self, x):
        self.data[self.cur] = x
        self.cur = (self.cur+1) % self.max
    def _full_get(self):
        return self.data[self.cur:]+self.data[:self.cur]
    def append(self, x):
        self.data.append(x)
        if len(self.data) == self.max:
            self.cur = 0
            # Permanently change self's methods from non-full to full
            self.append = self._full_appendself.tolist = self._full_get
    def tolist(self):
        return self.data

This method-switching approach is essentially equivalent to the class-switching one in the recipe's solution, albeit through rather different mechanisms. The best approach is probably to use class switching when all methods must be switched in bulk and method switching only when you need finer granularity of behavior change. Class switching is the only approach that works if you need to switch any special methods in a new-style class, since intrinsic lookup of special methods during various operations happens on the class, not on the instance (classic classes differ from new-style ones in this aspect).

You can use many other ways to implement a ring buffer. In Python 2.4, in particular, you should consider subclassing the new type collections.deque, which supplies a "double-ended queue", allowing equally effective additions and deletions from either end:

from collections import deque
class RingBuffer(deque):
    def _ _init_ _(self, size_max):
        deque._ _init_ _(self)
        self.size_max = size_max
    def append(self, datum):
        deque.append(self, datum)
        if len(self) > self.size_max:
            self.popleft( )
    def tolist(self):
        return list(self)

or, to avoid the if statement when at steady state, you can mix this idea with the idea of switching a method:

from collections import deque
class RingBuffer(deque):
    def _ _init_ _(self, size_max):
        deque._ _init_ _(self)
        self.size_max = size_max
    def _full_append(self, datum):
        deque.append(self, datum)
        self.popleft( )
    def append(self, datum):
        deque.append(self, datum)
        if len(self) == self.size_max:
            self.append = self._full_append
    def tolist(self):
        return list(self)

With this latest implementation, we need to switch only the append method (the tolist method remains the same), so method switching appears to be more appropriate than class switching.