Multiple inheritance: Cannot determine type of "X" in base class "Y" error

[sacha-senchuk]

This is probably more of a question than a real issue.

The following code is considered erroneous (tested in Python 3):

class BaseA:
    class Sub:
        pass

class BaseB:
    class Sub:
        pass

class Test(BaseA, BaseB):
    pass

mypy responds with:

…: error: Cannot determine type of 'Sub' in base class 'BaseA'
…: error: Cannot determine type of 'Sub' in base class 'BaseB'

But it doesn't complain if I don't use multiple inheritance:

class BaseA:
    class Sub:
        pass

class BaseB:
    class Sub:
        pass

class Test(BaseA):
    pass

[gvanrossum]

I think just the error is a bit confusing; in the M.I. case it is trying to point out that you are inheriting two unrelated class attributes named 'Sub'.

[JukkaL]

Based on the error message, it looks likely that the multiple inheritance validity check doesn't expect to see nested classes and gets confused.

[sacha-senchuk]

I have the impression that it gets confused, because the following code passes without errors:

class BaseA:
    sub = 1   

class BaseB:
    sub = 1 

class Test(BaseA, BaseB):
    pass

I don't really understand what's wrong with nested classes.

[gvanrossum]

Well, for one thing, two integers always have the same type. But two independently declared classes (even if they have the same name and contents) are not considered to have the same type.

[JukkaL]

Marked as a bug, because the error message is confusing.

[sacha-senchuk]

As a side note, I stumbled upon this issue by trying to use two different mixins in Django:

class CreatedDateMixin(models.Model):
    date_created = models.DateTimeField(_("created on"), auto_now_add=True)

    class Meta:
        abstract = True


class UpdatedDateMixin(models.Model):
    date_updated = models.DateTimeField(_("updated on"), auto_now=True)

    class Meta:
        abstract = True


class Foo(CreatedDateMixin, UpdatedDateMixin, models.Model):
    name = models.CharField(_("name"), max_length=255, blank=True)

# error: Cannot determine type of 'Meta' in base class 'CreatedDateMixin'
# error: Cannot determine type of 'Meta' in base class 'UpdatedDateMixin'

Not exactly a bug, but I don't know how to fix this… And I feel like others might easily stumble upon this issue as well.

[ilevkivskyi]

(Raising priority to high since this is a very common idiom in Django.)

[ilevkivskyi]

@mkurnikov you might want to try fixing this.

[ilevkivskyi]

Note that mypy still emits an error on the original example, because it is technically unsafe. Note however that @mkurnikov works on a mypy plugin for Django that will suppress this error for Django specific use cases.