'Displays a list of categories for all event source types, or,
if specified, for a specified source type. You can see a list
of the event categories and source types in the ` Events`_
topic in the Amazon RDS User Guide.
:type source_type: string
:param source_type: The type of source that will be generating the
events.
Valid values: db-instance | db-parameter-group | db-security-group |
db-snapshot'

def describe_event_categories(self, source_type=None):

    params = {}
    if (source_type is not None):
        params['SourceType'] = source_type
    return self._make_request(action='DescribeEventCategories', verb='POST', path='/', params=params)
