Additionaly, def increment(key): with lock(self.lock): value = self.counter_table.get(key) if not value: self.counter

Additionaly, def increment(key): with lock(self.lock): value = self.counter_table.get(key) if not value: self.counter_table.add(key, 1) value = 1 Should be replace by: def increment(self, key): with lock(self.lock): value = self.counter_table[key] = self.counter_table.setdefault(key, 0) + 1 return value UserDict.DictMixin has setdefault method, therefore it is reasonable to assume that a dictionary-like object should have it.