On 18 Mar 2022, at 14:37, Joao S. O. Bueno <jsbueno@python.org.br> wrote:

Please don’t toppost when responding to a normally threaded message. That makes it unnecessary hard to follow the conversation.


IMO this is a purism that have little, if any place in language restrictions.
I see that not allowing. "run once" iterables could indeed void attempts to write
"deliberatly non cooperative code" - but can it even be reliably detected? 

The other changes seem just to break backwards compatibility for little or no gain at all. 

It may not be worth the trouble to fix this, but Serhiy’s proposal does try to fix a ward.  

It may be better to rely on linter’s here, but one way to do this with few backward compatibility concerns:

- if __slots__ is a dict keep it as is
- Otherwise use tuple(__slots__) while constructing the class and store that value in the __slots__ attribute of the class

That way the value of the attribute reflects the slots that were created while not breaking code that uses __slots__ and doesn’t change the value after class creation.

Ronald




On Fri, Mar 18, 2022 at 6:57 AM Ronald Oussoren via Python-Dev <python-dev@python.org> wrote:


On 18 Mar 2022, at 10:29, Serhiy Storchaka <storchaka@gmail.com> wrote:

Currently __slots__ can be either string or an iterable of strings.

1. If it is a string, it is a name of a single slot. Third-party code which iterates __slots__ will be confused.

2. If it is an iterable, it should emit names of slots. Note that non-reiterable iterators are accepted too, but it causes weird bugs if __slots__ is iterated more than once. For example it breaks default pickling and copying.

I propose to restrict the type of __slots__. Require it always been a tuple of strings. Most __slots__ in real code are tuples. It is rarely we need only single slot and set __slots__ as a string.

It will break some code (there are 2 occurrences in the stdlib an 1 in scripts), but that code can be easily fixed.

Pydoc supports __slots__ that is a dict, and will use the values in the dict als documentation for the slots.   I’ve also seen code using ``__slots__ =  “field1 field2”.split()``. I don’t particularly like this code pattern, but your proposal would break this.

Also note that __slots__ only has a side effect during class definition, changing it afterwards is possible but has no effect (“class Foo: pass; Foo.__slots__ = 42”). This surprised my recently and I have no idea if this feature is ever used.

Ronald


—

Twitter / micro.blog: @ronaldoussoren

_______________________________________________
Python-Dev mailing list -- python-dev@python.org
To unsubscribe send an email to python-dev-leave@python.org
https://mail.python.org/mailman3/lists/python-dev.python.org/
Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/YQUWR7CYKNM65HR5FZQ3BANR5SNNK6N6/
Code of Conduct: http://python.org/psf/codeofconduct/

—

Twitter / micro.blog: @ronaldoussoren
Blog: https://blog.ronaldoussoren.net/