__slots__

[Ziga Seilnacht]

David Isaac wrote:
> 1. "Without a __dict__ variable,
> instances cannot be assigned new variables not listed in the __slots__
> definition."
>
> So this seemed an interesting restriction to impose in some instances,
> but I've noticed that this behavior is being called by some a side effect
> the reliance on which is considered unPythonic.  Why?

If you want to restrict  attribute asignment, you should use the
__setattr__ special method, see:
http://docs.python.org/ref/attribute-access.html

> 2. What is a simple example where use of slots has caused "subtle" problems,
> as some claim it will?

The first point is true only if all bases use __slots__:

>>> class A(object):
...     pass
...
>>> class B(A):
...     __slots__ = ('spam',)
...
>>> b = B()
>>> b.eggs = 1
>>> b.eggs
1

> 3. What is a simple example of a Pythonic use of __slots__ that does NOT
> involved the creation of **many** instances.
>
> Thanks,
> Alan Isaac

Ziga