Refactor a buffered class...

[lh84777 at yahoo.fr]

Hello,

i'm looking for this behaviour and i write a piece of code which works,
but it looks odd to me. can someone help me to refactor it ?

i would like to walk across a list of items by series of N (N=3 below)
of these. i had explicit mark of end of a sequence (here it is '.')
which may be any length and is composed of words.

for: s = "this . is a . test to . check if it . works . well . it looks
. like ."
the output should be (if grouping by 3) like:

=> this .
=> this . is a .
=> this . is a . test to .
=> is a . test to . check if it .
=> test to . check if it . works .
=> check if it . works . well .
=> works . well . it looks .
=> well . it looks . like .

my piece of code :

import sys

class MyBuffer:
    def __init__(self):
        self.acc = []
        self.sentries = [0, ]
    def append(self, item):
        self.acc.append(item)
    def addSentry(self):
        self.sentries.append(len(self.acc))
        print >> sys.stderr, "\t", self.sentries
    def checkSentry(self, size, keepFirst):
        n = len(self.sentries) - 1
        if keepFirst and n < size:
            return self.acc
        if n % size == 0:
            result = self.acc
            first = self.sentries[1]
            self.acc = self.acc[first:]
            self.sentries = [x - first for x in self.sentries]
            self.sentries.pop(0)
            return result

s = "this . is a . test to . check if it . works . well . it looks .
like ."
l = s.split()
print l

mb = MyBuffer()
n = 0
for x in l:
    mb.append(x)
    if x == '.':
        # end of something
        print "+", n
        n += 1
        mb.addSentry()
        current = mb.checkSentry(3, True) # GROUPING BY 3
        if current:
            print "=>", current