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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def GetNumberOfEventSources(self):
 """Retrieves the number event sources. Returns: int: number of event sources. """

    number_of_event_sources = self._CountStoredAttributeContainers(
        self._CONTAINER_TYPE_EVENT_SOURCE)

    number_of_event_sources += self._GetNumberOfSerializedAttributeContainers(
        self._CONTAINER_TYPE_EVENT_SOURCE)
    return number_of_event_sources