[Sqlalchemy-tickets] Issue #3420: LW KeyedTuples have instance dicts (ineffective __slots__) (zzzeek/sqlalchemy)

[Sebastian B. <iss...@bi...>]

New issue 3420: LW KeyedTuples have instance dicts (ineffective __slots__)
https://bitbucket.org/zzzeek/sqlalchemy/issue/3420/lw-keyedtuples-have-instance-dicts

Sebastian Bank:

```python
import sqlalchemy
import sqlalchemy.orm

engine = sqlalchemy.create_engine('sqlite://')
session = sqlalchemy.orm.Session(engine)
result = session.query(sa.func.now())[0]

result.__dict__  # should raise AttributeError

print('\n'.join('%s\t%s' % (getattr(cls, '__slots__', None), cls)
    for cls in result.__class__.mro()))
# ()	<class 'sqlalchemy.util._collections.result'>
# ()	<class 'sqlalchemy.util._collections._LW'>
# None	<class 'sqlalchemy.util._collections.AbstractKeyedTuple'>
# None	<type 'tuple'>
# None	<type 'object'>
```
Note that `__slots__` only works if [every class along the mro](https://docs.python.org/2/reference/datamodel.html#slots) spares instance `__dict__`s.
`__slots__ = ()` for `sqlalchemy.util._collections.AbstractKeyedTuple` shoud fix this.