Simple Criteria Filtering

criterion.py

from functools import singledispatch
from abc import ABCMeta, abstractmethod

class CriterionMeta(ABCMeta):
  
    def __new__(mcls, name, bases, attrs):
        dispatcher = singledispatch(attrs['_default'])
        attrs.update({'_dispatcher': dispatcher, 'register': dispatcher.register})
        return super(CriterionMeta, mcls).__new__(mcls, name, bases, attrs)

class Criterion(metaclass=CriterionMeta):
    @abstractmethod
    def _default(self, current_query):
        return current_query
    
    def __call__(self, repository, current_query):
        matcher = self._dispatcher.dispatch(repository)
        return matcher(self, current_query)

example.py

from collections import namedtuple
import operator
from criteria import Criterion

User = namedtuple('User', ['username', 'permission_level'])
Group = namedtuple('Group', ['name', 'perm_level'])


class PermissionLevel(Criterion):
    def __init__(self, permission_level, op=operator.eq):
        self.op = op
        self.permission_level = permission_level
    
    def _default(self, current_query):
        return filter(lambda obj: self.op(obj.permission_level, self.permission_level), current_query)
        

@PermissionLevel.register(Group)
def group_perm_filter(self, cq):
    return filter(lambda g: self.op(g.perm_level, self.permission_level), current_query)

IsAdmin = PermissionLevel(permission_level=1)

users = [User('fred', 0), User('hannah', 1)]
groups = [Group('Admins', 1), Group('Members', 0)]

list(IsAdmin(User, users)) # [User('hannah', 1)]
list(IsAdmin(Group, group)) # [Group('Admins', 1)]

fsqla_criterion.py

from example import PermissionLevel
from fsqla import db

class UserModel(Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.Unicode(32), unique=True)
    permission = db.Column(db.Integer)


@PermissionLevel.register(UserModel)
def user_model_permission_filter(self, current_query):
    return current_query.filter(self.op(UserModel.permission, self.permission_level))