[Python-Dev] Re: slots and default values (original) (raw)

Raymond Hettinger python@rcn.com
Tue, 13 May 2003 21:12:03 -0400

Aahz replies: > Why not do the initializing in init?

Michael: I presume that Raymond's concern was not that there wouldn't be a way to do initialization, but that this would become a new c.l.p FAQ and point of confusion for newbies. Unfortunately, I fear that it will.

Yes. Since it "works" with classic classes and unslotted newstyle classes, it isn't terribly unreasonable to believe it would work with slots. Further, there is no reason it couldn't work (either through an autoinit upon instance creation or through a default entry that references the class variable).

Michael: Already I am seeing that people are "discovering" class variables as a sort of "initialized instance variable" instead of using init as they "ought" to. Of course, it's NOT an initialized instance variable, but newbies stumble across it and seem to prefer it to using init.

Perhaps I skipped school the day they taught that was bad, but it seems perfectly reasonable to me and I'm sure it is a common practice. I even find it to be clearer and more maintainable than using init. The only downside I see is that self.classvar += 1 reads from and writes to a different place.

So, a reworded version of my question is "why not?". What is the downside of providing behavior that is similar to non-slotted classes? What is gained by these blocking an assignment and reporting a read-only error?

When I find an existing class can be made lighter by using slots, it would be nice to transform it with a single line. From:

class Tree(object): left = None right = None def init(self, value): self.value = value

adding only one line: slots = ('left', 'right', 'value')

It would be a bummer to also have to move the left/right = None into init and transform them into self.left = self.right = None.

Duncan Booth: The following works, but I can't remember whether you're supposed to be able to use a dict in slots or if it just happens to be allowed:

>>> class Pane(object): slots = { 'background': 'black', 'foreground': 'white', 'size': (80, 25) } def init(self): for k, v in self.slots.iteritems(): setattr(self, k, v)

slots accepts any iterable. So, yes, you're allowed eventhough that particular use was not intended.

There are several possible workarounds including metaclasses. My question is why there needs to be a workaround at all.

Thomas Heller: Unrelated to this topic, but Andrew's "What's new in Python 2.2" still presents slots as a way to constrain the instance variables:

A new-style class can define a class attribute named slots to constrain the list of legal attribute names.

Though I think of slots as a way to make lighter weight instances, constraining instance variables is also one of its functions.

Raymond Hettinger