@dataclass(slots=True) adds slots to dataclasses. It adds a slot per field. 
However, it doesn't account for slots already present in base classes:

>>> class Base:
...     __slots__ = ('a', )
...
>>> @dataclass(slots=True)
... class Foo(Base):
...     a: int
...     b: float
...
>>> Foo.__slots__
('a', 'b')  # should be: ('b', )


The __slots__ documentation says:

    If a class defines a slot also defined in a base class, the instance variable 
    defined by the base class slot is inaccessible (except by retrieving its descriptor 
    directly from the base class). This renders the meaning of the program undefined. 
    In the future, a check may be added to prevent this.

Solution: don't add slots which are already defined in any base classes:

>>> @dataclass
... class Bla(Base):
...     __slots__ = ('b', )
...     a: int
...     b: float
...
>>> Bla(4, 5.65)
Bla(a=4, b=5.65)

If you agree, I'd like to submit a PR to fix this. I already have a prototype working.