Generic slots classes don't work

[gabbard]

Attempting to create a generic attrs class with slots=True fails if it does not inherit from some other generic class.

from typing import TypeVar, Generic, Container
from unittest import TestCase

from attr import attrs

T = TypeVar('T')


class TestGenericBug(TestCase):
    def test_no_slots_ok(self):
        @attrs
        class Foo(Generic[T]):
            pass

    def test_no_attrs(self):
        class Meep(Generic[T]):
            __slots__ = ()

    def test_frozen_with_generic_parent_ok(self):
        @attrs(frozen=True)
        class Foo2(Generic[T], Container[T]):
            def __contains__(self, x: object) -> bool:
                return False
   
   # Failure is here
    def test_slots_with_no_parent(self):
        with self.assertRaisesRegex(TypeError, "Cannot inherit from plain Generic"):
            @attrs(slots=True)
            class Foo4(Generic[T]):
                pass

The full failure stack trace (if we remove the assertRaises is:

  File "/Users/gabbard/repos/isi-flexnlp/tests/util/test_generic_bug.py", line 27, in test_slots_with_no_parent
    class Foo4(Generic[T]):
  File "/Users/gabbard/anaconda3/envs/flexnlp/lib/python3.6/site-packages/attr/_make.py", line 637, in wrap
    return builder.build_class()
  File "/Users/gabbard/anaconda3/envs/flexnlp/lib/python3.6/site-packages/attr/_make.py", line 372, in build_class
    return self._create_slots_class()
  File "/Users/gabbard/anaconda3/envs/flexnlp/lib/python3.6/site-packages/attr/_make.py", line 442, in _create_slots_class
    cd,
  File "/Users/gabbard/anaconda3/envs/flexnlp/lib/python3.6/typing.py", line 948, in __new__
    raise TypeError("Cannot inherit from plain Generic")
TypeError: Cannot inherit from plain Generic

I am using attrs 17.3.0

[hynek]

This is something about meta classes, isn’t it? 😐

[dcbaker]

I ran into this also (on 18.1), and discovered that oddly enough defining __slots__ myself and using attr.s(these={...}) does work.

simplified example:

@attr.s(these={'foo': attr.ib()})
class Foo(Generic[T]):
     __slots__ = ['foo']

I have tested that slots work correctly in this case.

I don't really know what that means, but maybe that's useful for narrowing it down?

[leftys]

Confirming the bug in 18.2.0.

[dcbaker]

This does work with python 3.7, at least with __future__.annotations, I haven't tested it without.

[RazerM]

I used the workaround from @dcbaker, but you can use slots=True instead of defining it manually:

from typing import Generic, TypeVar
import attr

T = TypeVar('T')

@attr.s(these={'foo': attr.ib()}, slots=True)
class Foo(Generic[T]):
     pass

f = Foo(1)
assert not hasattr(f, '__dict__')
assert f.__slots__ == ('foo',)  # attrs 18.1
assert f.__slots__ == ('foo', '__weakref__')  # attrs 20.1

Edit: This only works on 3.7+ apparently