Using Protocols in django models raises metaclass conflict error

Question

Let's say I have PEP-544 protocol called Summable :

class Summable(Protocol):
    @property
    total_amount()-> Decimal:
      ...

And I have model Item that implements the Protocol

class Item(Summable, models.Model):
    discount = models.DecimalField(
        decimal_places=2,
        validators=[MaxValueValidator(1)],
        default=Decimal('0.00'),
        max_digits=10
    )
    price = models.DecimalField(
        decimal_places=4,
        validators=[MinValueValidator(0)],
        max_digits=10
    )

    @property
    def total_amount(self) - > Decimal:
       return self.price - self.price * self.discount

    class Meta:
        ordering = ['id']

I get:

TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

The same happens even if I extend Item's Meta from Summable.Meta and models.Model.Meta as well.

I am using python 3.9 Any ideas?

Answer 1

Well, there are many gotchas out there:

1. You need to create a new metaclass:

For Example:

class ModelProtocolMeta(type(Model), type(Protocol)):
     pass

2. You need to place the protocol last in order so that protocol doesn't overwrite the model's constructor with no_init. Protocol's no_init constructor is as follows:

def _no_init(self, *args, **kwargs):
    if type(self)._is_protocol:
        raise TypeError('Protocols cannot be instantiated')

so it would just overwite constuctor silently without any error since inherited class will have _is_protocol set to False

(note that super is not called, so we are talking about a complete overwrite)

so at the end of the day we need the following:

class Item(models.Model, Summable, metaclass=ModelProtocolMeta):
    discount = models.DecimalField(
        decimal_places=2,
        validators=[MaxValueValidator(1)],
        default=Decimal('0.00'),
        max_digits=10
    )
    price = models.DecimalField(
        decimal_places=4,
        validators=[MinValueValidator(0)],
        max_digits=10
    )

    @property
    def total_amount(self) -> Decimal:
       return sel.price - self.price * self.discount

Answer 2

This is similar to what Andreas did but I think a bit easier to use:

from typing import Protocol
from django.db import models
from typing import Any

django_model_type: Any = type(models.Model)
protocol_type: Any = type(Protocol)

class ModelProtocolMeta(django_model_type, protocol_type):
    """
    This technique allows us to use Protocol with Django models without metaclass conflict
    """
    pass

class Summable(Protocol):
    def total_amount(self) -> int: ...

class SummableModel(Summable, metaclass=ModelProtocolMeta): ...

And use it:

class Money(models.Model, SummableModel):
    def total_amount(self) -> int:
        return 100_000_000