<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def AddItem(self, item, f=lambda x: x):
 """Add an item to the ReservoirBucket, replacing an old item if necessary. The new item is guaranteed to be added to the bucket, and to be the last element in the bucket. If the bucket has reached capacity, then an old item will be replaced. With probability (_max_size/_num_items_seen) a random item in the bucket will be popped out and the new item will be appended to the end. With probability (1 - _max_size/_num_items_seen) the last item in the bucket will be replaced. Since the O(n) replacements occur with O(1/_num_items_seen) likelihood, the amortized runtime is O(1). Args: item: The item to add to the bucket. f: A function to transform item before addition, if it will be kept in the reservoir. """

    with self._mutex:
      if len(self.items) < self._max_size or self._max_size == 0:
        self.items.append(f(item))
      else:
        r = self._random.randint(0, self._num_items_seen)
        if r < self._max_size:
          self.items.pop(r)
          self.items.append(f(item))
        elif self.always_keep_last:
          self.items[-1] = f(item)
      self._num_items_seen += 1