A class with slots defined inherits from a class without slots defined

Question

I have the following code in Python 2.7:

>>> class A(object):
...     pass
... 
>>> class B(A):
...     __slots__ = tuple()
... 
>>> b = B()
>>> b.x = 1
>>> b.y = 2

Does that mean the __slots__ in the subclass B is basically useless, because it does not prevent dynamic attribute creation?

Related: [Python: How does inheritance of \_\_slots\_\_ in subclasses actually work?](http://stackoverflow.com/q/1816483) — Martijn Pieters, Jun 25 '15 at 21:01
And you should **not** rely on `__slots__` preventing dynamic attributes. That's not what `__slots__` are *for*. They are meant to limit the amount of memory instances require. — Martijn Pieters, Jun 25 '15 at 21:02
Hi, @jonsharpe. I have a followup question after reading the doc. The use of \__slots__ is to save memory, which is achieved by removing the dictionary \__dict__ for attribute storage. If the space allocated for the attributes that're assigned to \__slots__ is bigger than the space saved, does that mean setting \__slots__ is meaningless? — David Zheng, Jun 25 '15 at 21:39
That doesn't make sense; give that the same attribute values would be stored in the dictionary `__dict__` as you're storing using `__slots__`, how could that *ever* be the case? You're *always* saving the space that the dictionary itself would have taken up. Also note that I don't get notified if you spell my username wrongly! — jonrsharpe, Jun 25 '15 at 21:49
A dictionary object over-allocates and will *always* take up more space than the slots. — Martijn Pieters, Jun 25 '15 at 21:50
@jon but you always get notified for comments on your own posts. Which is why your name isn't even available for auto-completion and got misspelled in the first place :-) — Martijn Pieters, Jun 25 '15 at 21:52
@MartijnPieters yes, if the OP had commented on the answer I would have been pinged. Is autocomplete not available for editors' names, even though they're eligible for notifications? — jonrsharpe, Jun 25 '15 at 21:57
@jon: Ah, missed that this is *question* (I am on mobile at the moment). No, editor names are not available for auto-completion either. — Martijn Pieters, Jun 25 '15 at 22:00

score 2 · Accepted Answer · answered Jun 25 '15 at 20:58

2

Per the documentation:

When inheriting from a class without __slots__, the __dict__ attribute of that class will always be accessible, so a __slots__ definition in the subclass is meaningless.

answered Jun 25 '15 at 20:58

jonrsharpe

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A class with __slots__ defined inherits from a class without __slots__ defined

1 Answers1

A class with slots defined inherits from a class without slots defined