Data structure for topN

structures.py

class topN():
    """ Implements an efficient data structure to return the top N elements
    It increments the elements passed as parameter
    """

    def __init__(self, N=10):
        self.list = {}
        self.N = N
        self.topN = []

    def add(self, key):
        if key in self.list:
            self.list[key] += 1
        else:
            self.list[key] = 1

        self._updateTop(key)
        return

    def top(self):
        return self.topN

    def _updateTop(self, key):
        if key in self.topN:
            for i in range(len(self.topN)):
                if self.list[key] > self.list[self.topN[i]]:
                    posKey = self.topN.index(key)
                    self.topN[posKey], self.topN[i] = self.topN[i], self.topN[posKey]
                    break
        else:
            if len(self.topN) < self.N:
                self.topN.append(key)
            elif self.list[key] > self.list[self.topN[-1]]:
                self.topN[-1] = key