[issue27366] PEP487: Simpler customization of class creation

[Roundup Robot]

Roundup Robot added the comment:

New changeset 545bfa4c20eb by Victor Stinner in branch 'default':
Issue #27366: Fix init_subclass()
https://hg.python.org/cpython/rev/545bfa4c20eb

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

The new porting note doesn't quite capture all the subtleties of the situation, 
but should be sufficient for folks to get any affected custom metaclasses 
working again (since the key is to avoid passing those parameters up to 
type.__new__).

I also added a note about the fact that __init_subclass__ implementations don't 
have access to the metaclass hint, but can retrieve the actual metaclass based 
on the passed in class object.

--
resolution:  -> fixed
stage: needs patch -> resolved
status: open -> closed

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[issue27366] PEP487: Simpler customization of class creation

[Roundup Robot]

Roundup Robot added the comment:

New changeset 313e8fdb0d0c by Nick Coghlan in branch 'default':
Issue 27366: PEP 487 docs updates
https://hg.python.org/cpython/rev/313e8fdb0d0c

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

D'oh, another good catch Emanuel - and I'm even the one that raised the need to 
mention that in the Porting notes during the python-dev discussion :P

I'll post an update to the What's New shortly.

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[issue27366] PEP487: Simpler customization of class creation

[Emanuel Barry]

Emanuel Barry added the comment:

Hello Martin, and thank you for your patch! However, this patch removed the 
ability to pass keyword arguments to `type`, and it's not documented anywhere. 
I believe it should be documented at least in e.g. the "Changes in the Python 
API" of the What's New in Python 3.6 document. Anyone cares to submit a patch?

--
nosy: +ebarry
resolution: fixed -> 
status: closed -> open

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[issue27366] PEP487: Simpler customization of class creation

[Berker Peksag]

Berker Peksag added the comment:

Thanks for the review, Nick! (and also thanks to Martin for the great PEP!)

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[issue27366] PEP487: Simpler customization of class creation

[Roundup Robot]

Roundup Robot added the comment:

New changeset 8747e3455ecb by Berker Peksag in branch 'default':
Issue #27366: Tweak PEP 487 documentation
https://hg.python.org/cpython/rev/8747e3455ecb

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

Good catches Berker - go ahead and apply the improvements whenever's convenient 
:)

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[issue27366] PEP487: Simpler customization of class creation

[Berker Peksag]

Berker Peksag added the comment:

Sorry, I'm a bit late to the party. Here are some tweaks to ecc7bff738e0. I've 
added versionadded directives, updated the tests to use new style classes and 
removed a duplicate sentence from the __init_subclass__ docstring.

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Added file: http://bugs.python.org/file43946/issue27366_tweaks.diff

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

Thanks once again Martin, especially for your patience with the long process in 
getting this proposal all the way through to resolution :)

I mostly applied your patch as-is, but tweaked a few aspects of the 
documentation before committing it (mainly expanding the What's New entry, and 
showing a few more of the moving parts in the  __init_subclass__ example).

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue27366] PEP487: Simpler customization of class creation

[Roundup Robot]

Roundup Robot added the comment:

New changeset ecc7bff738e0 by Nick Coghlan in branch 'default':
Issue #27366: Implement PEP 487
https://hg.python.org/cpython/rev/ecc7bff738e0

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nosy: +python-dev

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

Martin's latest patch looks good to me, so I'm applying it now and will push it 
once a local run of the test suite gives it the thumbs up :)

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[issue27366] PEP487: Simpler customization of class creation

[Guido van Rossum]

Changes by Guido van Rossum :


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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Martin Teichmann added the comment:

I looked over the patch once more and found some places where
I didn't follow normal coding standards. I changed that, namely
now the code returns -1 meaning an exception happened and 0 on
success, which is what many functions in typeobject.c do.

So I think this patch should be ready.

--
Added file: http://bugs.python.org/file43882/pep487.patch

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[issue27366] PEP487: Simpler customization of class creation

[Guido van Rossum]

Guido van Rossum added the comment:

That's a no from me.

On Sunday, July 24, 2016, Martin Teichmann  wrote:

>
> Martin Teichmann added the comment:
>
> Thanks for the nice review!
>
> I made the proposed changes, see attached patch.
>
> I am still waiting for a decision whether type.__setattr__ should call
> __set_name__ from python-dev, once that's sorted out I'll implement and
> test the one or the other behavior.
>
> --
> Added file: http://bugs.python.org/file43854/pep487.patch
>
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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Martin Teichmann added the comment:

Thanks for the nice review!

I made the proposed changes, see attached patch.

I am still waiting for a decision whether type.__setattr__ should call 
__set_name__ from python-dev, once that's sorted out I'll implement and test 
the one or the other behavior.

--
Added file: http://bugs.python.org/file43854/pep487.patch

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

Martin's current implementation basically looks good to me - I've mainly 
suggested some changes to the documentation and additional test cases that help 
stress test some of the more complex inheritance hierarchies described above, 
although there are a few other other suggestions as well.

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Changes by Nick Coghlan :


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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

Scratch that, my revised idea is fundamentally broken, as this example 
illustrates:

>>> class BaseClass: pass
... 
>>> class OtherClass: pass
... 
>>> class SubClass(OtherClass, BaseClass): pass
... 
>>> SubClass.mro()
[, , , ]


The PEP as written handles this correctly, while the alternative signature 
would fail miserably ("OtherClass" wouldn't chain up to "BaseClass" properly). 
So I'll review the rest of the patch, and we can figure out the documentation 
problem later.

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Nick Coghlan added the comment:

I started reviewing Martin's patch, and I initially thought I had found a 
problem with the way __init_subclass__ is currently defined. It turned out I 
was wrong about it actually being broken, but I *do* now think it's inherently 
confusing, and we may be able to do something different that's more obviously 
correct (or at least easier to document - it was proposing revisions to the 
documentation that got me thinking along this path).

Specifically, I was thinking using super() in either the zero argument form or 
the explicit form could create an infinite loop due to the way we're currently 
proposing to interact with the MRO. Consider:

class BaseClass:
@classmethod
def __init_subclass__(cls):
super(cls, BaseClass).__init_subclass__()

class SubClass(BaseClass):
pass

If the initial call made by type.__new__() is effectively 
"SubClass.mro()[1].__init_subclass__()", then the super() call is going to call 
BaseClass.__init_subclass__ again.

However, it turned out I was wrong, as that's not what happens: the call made 
by the type machinery is instead "super(SubClass, SubClass).__init_subclass__", 
which gets it to the right place in the MRO and causes further super() calls to 
do the right thing.

However, the "more obviously correct" signature that occurred to me was to do 
this instead:

class BaseClass:
@classmethod
def __init_subclass__(cls, subcls):
super(cls, BaseClass).__init_subclass__(subcls)

class SubClass(BaseClass):
pass

Then the invocation from type.__new__ could be defined more simply as:

SubClass.mro()[1].__init_subclass__(SubClass)

In all cases then (regardless of where you were in the MRO), "cls" would refer 
to "the class first in the MRO after the class being defined" and "subcls" 
would refer to "the class currently being defined".

If you consider the plugin example in the PEP, with the revised signature, it 
would look like:

class PluginBase:
subclasses = []

def __init_subclass__(cls, subcls, **kwargs):
super().__init_subclass__(**kwargs)
cls.subclasses.append(subcls)

And *even if the subclass being defined shadowed the "subclasses" attribute*, 
this initialisation would still work. (You can still get yourself in trouble if 
a subclass somewhere else in the MRO shadows the attribute, but that's life in 
complex type hierarchies)

In the version in the PEP, the fact that "cls" is actually a subclass, and 
we're relying on the MRO to find "subclasses" is a really subtle implementation 
detail, while having two parameters makes it clear that "the class defining 
__init_subclass__" is distinct from the "new subclass being defined".

--
nosy: +gvanrossum

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[issue27366] PEP487: Simpler customization of class creation

[Nick Coghlan]

Changes by Nick Coghlan :


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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Changes by Martin Teichmann :


Added file: http://bugs.python.org/file43835/pep487.patch

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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Changes by Martin Teichmann :


Removed file: http://bugs.python.org/file43834/pep487.patch

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[issue27366] PEP487: Simpler customization of class creation

[Berker Peksag]

Changes by Berker Peksag :


--
components: +Interpreter Core -Library (Lib)
nosy: +berker.peksag
stage:  -> patch review

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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Changes by Martin Teichmann :


Added file: http://bugs.python.org/file43834/pep487.patch

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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Changes by Martin Teichmann :


Removed file: http://bugs.python.org/file43611/pep487a.patch

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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Changes by Martin Teichmann :


Removed file: http://bugs.python.org/file43506/pep487.patch

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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

Martin Teichmann added the comment:

This is a C implementation of PEP 487, directly in the Python core

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Added file: http://bugs.python.org/file43611/pep487a.patch

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[issue27366] PEP487: Simpler customization of class creation

[Martin Teichmann]

New submission from Martin Teichmann:

This is the implementation of PEP 487.

It adds a metaclass to types that calls a method on a class
once it is subclassed. This way one can customize the creation
of classes without the need to write an own metaclass.

As a second functionality, it calls a method on each descriptor
in a class, such that descriptors know their name.

--
components: Library (Lib)
files: pep487.patch
keywords: patch
messages: 269050
nosy: Martin.Teichmann
priority: normal
severity: normal
status: open
title: PEP487: Simpler customization of class creation
type: enhancement
versions: Python 3.6
Added file: http://bugs.python.org/file43506/pep487.patch

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