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

[Eryk Sun]

On 3/19/22, Eryk Sun  wrote:
> On 3/18/22, Ronald Oussoren via Python-Dev  wrote:
>>
>> - 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
>
> If this is just for pydoc, then it can be updated with new behavior.
> For example, if the given __slots__ is a dict, set it as something
> like __slots_doc__, and rewrite __slots__ as a tuple of the keys.

Or extend pydoc to support a mappingproxy for __slots__.
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[Python-Dev] Re: Restrict the type of __slots__

[Eryk Sun]

On 3/18/22, Ronald Oussoren via Python-Dev  wrote:
>
> - 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

If this is just for pydoc, then it can be updated with new behavior.
For example, if the given __slots__ is a dict, set it as something
like __slots_doc__, and rewrite __slots__ as a tuple of the keys.
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[Python-Dev] Re: Restrict the type of __slots__

[Alex Waygood]

I'm +1 on disallowing strings specifically. The unexpected behaviour that 
ensues if you try to iterate over a class's slots is a footgun that I've 
trodden on a few times, and it's always been annoying.

But I don't see any particular reason to disallow any other kind of iterable, 
and I'm definitely -1 on disallowing using a dictionary for __slots__. Using a 
dictionary for __slots__ as a way to document attributes is a feature I very 
much like.

Best,
Alex

> On 18 Mar 2022, at 20:59, Guido van Rossum  wrote:
> 
> 
>> On Fri, Mar 18, 2022 at 9:40 AM Paul Bryan  wrote:
>>> On Fri, 2022-03-18 at 09:35 -0700, Guido van Rossum wrote:
>>> The motivation has been explained already.
>> 
>> In this thread?
> 
> Yes, Eric's message.
>  
>>> What on earth did your test do that got a speedup by using sets? Was it 
>>> repeatedly checking whether a variable was in a slot? The other thing this 
>>> does is rearrange the order in which slots appear from run to run (since 
>>> set order is affected by hash randomization) and you might have gotten 
>>> lucky with a popular attribute being moved to the front, where it's more 
>>> likely to be in the memory cache already (cache lines being 64 bytes and 
>>> pointers being 8 bytes nowadays).
>> 
>> I created objects in a tight loop, populating attributes, noting the elapsed 
>> time.
> 
> It does not make sense that that is correlated to the type of __slots__, 
> since __slots__ is not used for instance creation at all (it is only used to 
> create the class). I stick to my likely explanation.
> 
> Regarding Serhiy's proposal, I'm +0 on disallowing strings, and +0 on 
> disallowing things that can't be reiterated (but it's not a problem in 
> practice). Given other responses the status quo is likely best.
> 
> -- 
> --Guido van Rossum (python.org/~guido)
> Pronouns: he/him (why is my pronoun here?)
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[Python-Dev] Re: Restrict the type of __slots__

[Eric V. Smith]

On 3/18/2022 4:58 PM, Guido van Rossum wrote:

On Fri, Mar 18, 2022 at 9:40 AM Paul Bryan  wrote:

On Fri, 2022-03-18 at 09:35 -0700, Guido van Rossum wrote:

The motivation has been explained already.


In this thread?


Yes, Eric's message.


What on earth did your test do that got a speedup by using sets?
Was it repeatedly checking whether a variable was in a slot? The
other thing this does is rearrange the order in which slots
appear from run to run (since set order is affected by hash
randomization) and you might have gotten lucky with a popular
attribute being moved to the front, where it's more likely to be
in the memory cache already (cache lines being 64 bytes and
pointers being 8 bytes nowadays).


I created objects in a tight loop, populating attributes, noting
the elapsed time.


It does not make sense that that is correlated to the type of 
__slots__, since __slots__ is not used for instance creation at all 
(it is only used to create the class). I stick to my likely explanation.


Regarding Serhiy's proposal, I'm +0 on disallowing strings, and +0 on 
disallowing things that can't be reiterated (but it's not a problem in 
practice). Given other responses the status quo is likely best.


The PR for the issue I mentioned (https://bugs.python.org/issue46382) 
allows strings and iterables (but not iterators). And it uses a match 
statement, no less! I think that's good enough for dataclasses.


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

[Guido van Rossum]

On Fri, Mar 18, 2022 at 9:40 AM Paul Bryan  wrote:

> On Fri, 2022-03-18 at 09:35 -0700, Guido van Rossum wrote:
>
> The motivation has been explained already.
>
>
> In this thread?
>

Yes, Eric's message.


> What on earth did your test do that got a speedup by using sets? Was it
> repeatedly checking whether a variable was in a slot? The other thing this
> does is rearrange the order in which slots appear from run to run (since
> set order is affected by hash randomization) and you might have gotten
> lucky with a popular attribute being moved to the front, where it's more
> likely to be in the memory cache already (cache lines being 64 bytes and
> pointers being 8 bytes nowadays).
>
>
> I created objects in a tight loop, populating attributes, noting the
> elapsed time.
>

It does not make sense that that is correlated to the type of __slots__,
since __slots__ is not used for instance creation at all (it is only used
to create the class). I stick to my likely explanation.

Regarding Serhiy's proposal, I'm +0 on disallowing strings, and +0 on
disallowing things that can't be reiterated (but it's not a problem in
practice). Given other responses the status quo is likely best.

-- 
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*

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

[Steve Dower]

On 3/18/2022 2:38 PM, Eric V. Smith wrote:
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 seems totally sufficient for dataclasses to require that base class 
__slots__ are a string or iterables of strings. Which is apparently what 
would happen today, since anything else would cause incorrect behaviour.


A single special case doesn't have to become a language-wide backwards 
compatibility break, even if it perhaps would've been better to have had 
the restriction from the start.


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

[Raymond Hettinger]

> I propose to restrict the type of __slots__.

-1 for adding a restriction.  This breaks code for no good reason.  This
API has been around for a very long time.  I've seen lists, tuples, dicts,
single strings, and occasionally something more exotic.  Why wreck stable
code?

Also, the inspect module will detect whether __slots__ is a dictionary and
will use it to display docstrings.  In the database world, data
dictionaries have proven value, so it would be a bummer to kill off this
functionality which is used in much the same way as docstrings for
properties.  It is still rarely used, but I'm hoping it will catch on (just
like people are slowly growing more aware that they can add docstringa to
fields in named tuples).

Raymond



On Fri, Mar 18, 2022 at 4:33 AM Serhiy Storchaka 
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.
>
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[Python-Dev] Re: Restrict the type of __slots__

[Paul Bryan]

On Fri, 2022-03-18 at 09:35 -0700, Guido van Rossum wrote:
> The motivation has been explained already.

In this thread?

> What on earth did your test do that got a speedup by using sets? Was
> it repeatedly checking whether a variable was in a slot? The other
> thing this does is rearrange the order in which slots appear from run
> to run (since set order is affected by hash randomization) and you
> might have gotten lucky with a popular attribute being moved to the
> front, where it's more likely to be in the memory cache already
> (cache lines being 64 bytes and pointers being 8 bytes nowadays).

I created objects in a tight loop, populating attributes, noting the
elapsed time.

> I agree that dicts are a use case to preserve.
> 
> On Fri, Mar 18, 2022 at 08:59 Paul Bryan  wrote:
> > You've proposed a "what", but I don't see a "why".
> > 
> > Indeed, it will break some code.
> > 
> > I've been (currently legally) expressing __slots__ as sets, which
> > is arguably more consistent with its purpose, and in testing
> > appeared to perform better than tuples.
> > 
> > So, I would request that you amend the allowed types to include
> > sets.
> > 
> > Also, people are using dicts to provide docstrings on attributes,
> > so that would need to be addressed as well. 
> > 
> > On Fri, 2022-03-18 at 11:42 +0200, Serhiy Storchaka wrote:
> > > 18.03.22 11:29, Serhiy Storchaka пише:
> > > > It will break some code (there are 2 occurrences in the stdlib
> > > > an 1 in 
> > > > scripts), but that code can be easily fixed.
> > > 
> > > Actually it will break more code, because initializing __slots__
> > > with a 
> > > list is pretty common. Maybe restrict it to tuple or list? But
> > > having an 
> > > immutable __slots__ may be more reliable.
> > > 
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[Python-Dev] Re: Restrict the type of __slots__

[Guido van Rossum]

The motivation has been explained already.

What on earth did your test do that got a speedup by using sets? Was it
repeatedly checking whether a variable was in a slot? The other thing this
does is rearrange the order in which slots appear from run to run (since
set order is affected by hash randomization) and you might have gotten
lucky with a popular attribute being moved to the front, where it's more
likely to be in the memory cache already (cache lines being 64 bytes and
pointers being 8 bytes nowadays).

I agree that dicts are a use case to preserve.

On Fri, Mar 18, 2022 at 08:59 Paul Bryan  wrote:

> You've proposed a "what", but I don't see a "why".
>
> Indeed, it will break some code.
>
> I've been (currently legally) expressing __slots__ as sets, which is
> arguably more consistent with its purpose, and in testing appeared to
> perform better than tuples.
>
> So, I would request that you amend the allowed types to include sets.
>
> Also, people are using dicts to provide docstrings on attributes, so that
> would need to be addressed as well.
>
> On Fri, 2022-03-18 at 11:42 +0200, Serhiy Storchaka wrote:
>
> 18.03.22 11:29, Serhiy Storchaka пише:
>
> It will break some code (there are 2 occurrences in the stdlib an 1 in
> scripts), but that code can be easily fixed.
>
>
> Actually it will break more code, because initializing __slots__ with a
> list is pretty common. Maybe restrict it to tuple or list? But having an
> immutable __slots__ may be more reliable.
>
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[Python-Dev] Re: Restrict the type of __slots__

[Paul Bryan]

You've proposed a "what", but I don't see a "why".

Indeed, it will break some code.

I've been (currently legally) expressing __slots__ as sets, which is
arguably more consistent with its purpose, and in testing appeared to
perform better than tuples.

So, I would request that you amend the allowed types to include sets.

Also, people are using dicts to provide docstrings on attributes, so
that would need to be addressed as well. 

On Fri, 2022-03-18 at 11:42 +0200, Serhiy Storchaka wrote:
> 18.03.22 11:29, Serhiy Storchaka пише:
> > It will break some code (there are 2 occurrences in the stdlib an 1
> > in 
> > scripts), but that code can be easily fixed.
> 
> Actually it will break more code, because initializing __slots__ with
> a 
> list is pretty common. Maybe restrict it to tuple or list? But having
> an 
> immutable __slots__ may be more reliable.
> 
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[Python-Dev] Re: Restrict the type of __slots__

[Eric V. Smith]

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




On 18 Mar 2022, at 14:37, Joao S. O. Bueno  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 
 wrote:





On 18 Mar 2022, at 10:29, Serhiy Storchaka 
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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[Python-Dev] Re: Restrict the type of __slots__

[Ronald Oussoren via Python-Dev]

> On 18 Mar 2022, at 14:37, Joao S. O. Bueno  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 
> mailto:python-dev@python.org>> wrote:
> 
> 
>> On 18 Mar 2022, at 10:29, Serhiy Storchaka > > 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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[Python-Dev] Re: Restrict the type of __slots__

[Joao S. O. Bueno]

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.



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  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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>
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>
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> Blog: https://blog.ronaldoussoren.net/
>
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[Python-Dev] Re: Restrict the type of __slots__

[Ronald Oussoren via Python-Dev]

> On 18 Mar 2022, at 10:29, Serhiy Storchaka  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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[Python-Dev] Re: Restrict the type of __slots__

[Serhiy Storchaka]

18.03.22 11:29, Serhiy Storchaka пише:
It will break some code (there are 2 occurrences in the stdlib an 1 in 
scripts), but that code can be easily fixed.


Actually it will break more code, because initializing __slots__ with a 
list is pretty common. Maybe restrict it to tuple or list? But having an 
immutable __slots__ may be more reliable.


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