def process(self):
    for msg in self.messages:
        for key in self.fields:
            value = msg.get(key, None)
            if (value is not None):
                self.counters[key][value] += 1
        (yield msg)
def process(self):
    for msg in self.messages:
        for key in self.fields:
            value = msg.get(key, None)
            if (value is not None):
                self.counters[key][value] += 1
        (yield msg)