'Displays a list of event categories for all event source
types, or for a specified source type. For a list of the event
categories and source types, go to `Amazon Redshift Event
Notifications`_.
:type source_type: string
:param source_type: The source type, such as cluster or parameter
group, to which the described event categories apply.
Valid values: cluster, snapshot, parameter group, and security group.'

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)
