django_bitfield.py

from django import forms
from django.db import  models


class BitFieldWrapper:

    def __init__(self, instance, field, value):
        self.instance = instance
        self.field = field
        self.value = value

    def has(self, bit):
        return self.value &  1 << (bit - 1)

    def set(self, bit):
        self.value = self.value |  1 << (bit - 1)

    def all(self):
        return [choice[0] for choice in self.field.bit_choices if self.has(choice[0])]

    def __int__(self):
        return self.value

    def __str__(self):
        return str(self.value)


class BitDescriptor:

    def __init__(self, field):
        self.field = field

    def __get__(self, instance, cls=None):
        if instance is None:
            return self

        value = instance.__dict__[self.field.name]

        if value is None:
            value = self.field.default

        if isinstance(value, int) and not isinstance(value, BitFieldWrapper):
            return self.field.attr_class(instance, self.field, value)

        return instance.__dict__[self.field.name]

    def __set__(self, instance, value):
        instance.__dict__[self.field.name] = value


class BitField(models.IntegerField):
    attr_class = BitFieldWrapper
    descriptor_class = BitDescriptor
    # https://github.com/kelvinwong-ca/django-select-multiple-field
    # https://github.com/goinnn/django-multiselectfield
    # https://djangosnippets.org/snippets/2753/
    # https://docs.djangoproject.com/en/1.10/ref/forms/fields/
    # https://docs.djangoproject.com/en/1.10/ref/models/fields/#commaseparatedintegerfield

    def __init__(self, bit_choices=None, *args, **kwargs):
        self.bit_choices = bit_choices
        super().__init__(*args, **kwargs)


    def contribute_to_class(self, cls, name, **kwargs):
        super().contribute_to_class(cls, name, **kwargs)
        setattr(cls, self.name, self.descriptor_class(self))



def test_bitfieldwrapper():
    choices = (
        (1, 'a'),
        (2, 'b'),
        (3, 'c'),
        (4, 'd'),
    )
    wrapper = BitFieldWrapper(None, BitField(bit_choices=choices), 0)
    assert int(wrapper) == 0
    wrapper.set(1)
    assert int(wrapper) == 1

    wrapper = BitFieldWrapper(None, BitField(bit_choices=choices), 0)
    wrapper.set(2)
    assert int(wrapper) == 2
    wrapper.set(1)
    assert int(wrapper) == 3
    wrapper.set(3)
    assert int(wrapper) == 7

    set(wrapper.all()) == {1, 2}