Django rest framework group based permissions for individual views

I am using DRF for writing API's. I would like to give different permissions for each view in my Modelviewsets. I have two groups(customers and staff). I have filtered them as Isstaff and Iscustomer in permissions.py.

class Iscustomer(permissions.BasePermission):
    def has_permission(self, request, view):
        if request.user and request.user.groups.filter(name='customers'):
            return True
        return False


class Isstaff(permissions.BasePermission):
    def has_permission(self, request, view):
        if request.user and request.user.groups.filter(name='staff'):
            return True
        return False

I am trying to overide using get_permissions method. When I put a single group in self.permission_classes, it works fine.

class cityviewset(viewsets.ModelViewSet):
    queryset = city.objects.all()
    serializer_class = citySerializer

    def get_permissions(self):    
        if self.request.method == 'POST' or self.request.method == 'DELETE':
            self.permission_classes = [Isstaff]
        return super(cityviewset, self).get_permissions()

But, when i try to put multiple groups in self.permission_classes, it fails.

def get_permissions(self):
    if self.request.method == 'POST' or self.request.method == 'DELETE':
        self.permission_classes = [Isstaff,Iscustomer,]
    return super(cityviewset, self).get_permissions()