[Python-Dev] Re: Restrict the type of __slots__

2022年3月18日 07:43:25 -0700 

On 3/18/2022 10:01 AM, Ronald Oussoren via Python-Dev wrote:

On 18 Mar 2022, at 14:37, Joao S. O. Bueno <[email protected]> wrote:
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.
There a practical need for these changes. See https://bugs.python.org/issue46382 for a case where dataclasses needs __slots__ to be iterated over after the class has been created. And it would be good if __slots__ accurately reflected the slots that were actually created.
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. 
I like this approach, too.
Eric

Ronald
On Fri, Mar 18, 2022 at 6:57 AM Ronald Oussoren via Python-Dev <[email protected]> wrote: 
 On 18 Mar 2022, at 10:29, Serhiy Storchaka <[email protected]>
 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

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