filter_by = fields.Selection(selection=_get_filter_possible_values, string='Filter By')

@api.model
@api.onchange('report_type')
def _get_filter_possible_values(self):
    # selection = [()]
    if str(self.report_type) == 'child_nationality':
        return [('nature', _('Nature of Assault')),
            ('state', _('Case Status')),
            ('area', _('Area')),
            ]
    elif str(self.report_type) == 'assault_age_groups':
        return [('nature', _('Nature of Assault')),
            ('state', _('Case Status')),
            ('gender', _('Gender')),
            ('area', _('Area')),
            ]
    elif str(self.report_type) == 'gender':
        return [('area', _('Area')),
            ]
    else:

        return [('nature', _('Nature of Assault')),
            ('state', _('Case Status')),
            ]

selection box shows always in the else condition 

how can i solve this ?

Thanks in Advance