'Get all parameters associated with a ParameterGroup
:type groupname: str
:param groupname: The name of the DBParameter group to retrieve.
:type source: str
:param source: Specifies which parameters to return.
If not specified, all parameters will be returned.
Valid values are: user|system|engine-default
:type max_records: int
:param max_records: The maximum number of records to be returned.
If more results are available, a MoreToken will
be returned in the response that can be used to
retrieve additional records.  Default is 100.
:type marker: str
:param marker: The marker provided by a previous request.
:rtype: :class:`boto.ec2.parametergroup.ParameterGroup`
:return: The ParameterGroup'

def get_all_dbparameters(self, groupname, source=None, max_records=None, marker=None):

    params = {'DBParameterGroupName': groupname}
    if source:
        params['Source'] = source
    if max_records:
        params['MaxRecords'] = max_records
    if marker:
        params['Marker'] = marker
    pg = self.get_object('DescribeDBParameters', params, ParameterGroup)
    pg.name = groupname
    return pg
