def update(self, instance, oldValue, newValue, subKey):
    """Updates the aggregate based on a change in the child value."""
    histogram = self.__get__(instance, None)
    histogram[subKey] += newValue - oldValue
def update ( self , instance , oldValue , newValue , subKey ) : histogram = self . __get__ ( instance , None ) histogram [ subKey ] += newValue - oldValue
Updates the aggregate based on a change in the child value.