Re: [Python-Dev] PEP487: Simpler customization of class creation
Hello, there has been quite some discussion on why PEP 487's __init_subclass__ initializes subclasses, and not the class itself. I think the most important details have been already thoroughly discussed here. One thing I was missing in the discussion is practical examples. I have been using PEP 487-like metaclasses since several years now, and I have never come across an application where it would have even been nice to initialize the class itself. Also, while researching other people's code when I wrote PEP 487, I couldn't find any such code elsewhere, yet I found a lot of code where people took the wildest measure to prevent a metaclass in doing its job on the first class it is used for. One example is enum.EnumMeta, which contains code not to make enum.Enum an enum (I do not try to propose that the enum module should use PEP 487, it's way to complicated for that). So once we have a practical example, we could start discussing how to mitigate the problem. Btw, everyone is still invited to review the patch for PEP 487 at http://bugs.python.org/issue27366. Many thanks to Nick who already reviewed, and also to Guido who left helpful comments! Greetings Martin ___ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP487: Simpler customization of class creation
In the traitlets library I mentioned earlier, we do have a need for this.
The corresponding function is called `setup_class`.
What it does is setting some class attributes that are required for certain
types of descriptors to be able to initialize themselves.
class MetaHasTraits(MetaHasDescriptors):
"""A metaclass for HasTraits."""
def setup_class(cls, classdict):
cls._trait_default_generators = {}
super(MetaHasTraits, cls).setup_class(classdict)
Sylvain
On Fri, Jul 29, 2016 at 5:01 PM, Martin Teichmann
wrote:
> Hello,
>
> there has been quite some discussion on why PEP 487's
> __init_subclass__ initializes subclasses, and not the class itself. I
> think the most important details have been already thoroughly
> discussed here.
>
> One thing I was missing in the discussion is practical examples. I
> have been using PEP 487-like metaclasses since several years now, and
> I have never come across an application where it would have even been
> nice to initialize the class itself. Also, while researching other
> people's code when I wrote PEP 487, I couldn't find any such code
> elsewhere, yet I found a lot of code where people took the wildest
> measure to prevent a metaclass in doing its job on the first class it
> is used for. One example is enum.EnumMeta, which contains code not to
> make enum.Enum an enum (I do not try to propose that the enum module
> should use PEP 487, it's way to complicated for that).
>
> So once we have a practical example, we could start discussing how to
> mitigate the problem.
>
> Btw, everyone is still invited to review the patch for PEP 487 at
> http://bugs.python.org/issue27366. Many thanks to Nick who already
> reviewed, and also to Guido who left helpful comments!
>
> Greetings
>
> Martin
> ___
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[Python-Dev] Summary of Python tracker Issues
ACTIVITY SUMMARY (2016-07-22 - 2016-07-29) Python tracker at http://bugs.python.org/ To view or respond to any of the issues listed below, click on the issue. Do NOT respond to this message. Issues counts and deltas: open5588 ( +2) closed 33818 (+56) total 39406 (+58) Open issues with patches: 2442 Issues opened (41) == #24773: Implement PEP 495 (Local Time Disambiguation) http://bugs.python.org/issue24773 reopened by martin.panter #27593: Deprecate sys._mercurial and create sys._git http://bugs.python.org/issue27593 opened by brett.cannon #27594: Assertion failure when running "test_ast" tests with coverage. http://bugs.python.org/issue27594 opened by ap #27595: Document PEP 495 (Local Time Disambiguation) features http://bugs.python.org/issue27595 opened by belopolsky #27596: Build failure with Xcode 8 beta on OSX 10.11 http://bugs.python.org/issue27596 opened by ronaldoussoren #27597: Add usage examples for TracebackException, StackSummary and Fr http://bugs.python.org/issue27597 opened by cool-RR #27598: Add SizedIterable to collections.abc and typing http://bugs.python.org/issue27598 opened by brett.cannon #27599: Buffer overrun in binascii http://bugs.python.org/issue27599 opened by serhiy.storchaka #27602: Enable py launcher to launch repository Python. http://bugs.python.org/issue27602 opened by terry.reedy #27603: Migrate IDLE context menu items to shell extension http://bugs.python.org/issue27603 opened by steve.dower #27604: More details about `-O` flag http://bugs.python.org/issue27604 opened by cool-RR #27605: Inconsistent calls to __eq__ from built-in __contains__ http://bugs.python.org/issue27605 opened by vfaronov #27606: Android cross-built for armv5te with clang and '-mthumb' crash http://bugs.python.org/issue27606 opened by xdegaye #27607: Importing the main module twice leads to two incompatible inst http://bugs.python.org/issue27607 opened by SylvieLorxu #27609: IDLE completions: format, factor, and fix http://bugs.python.org/issue27609 opened by terry.reedy #27611: test_tix cannot import _default_root after test_idle http://bugs.python.org/issue27611 opened by martin.panter #27612: socket.gethostbyname resolving octal IP addresses incorrectly http://bugs.python.org/issue27612 opened by mattrobenolt #27613: Empty iterator is rendered as a single bracket ] when using js http://bugs.python.org/issue27613 opened by altvod #27614: Race in test_docxmlrpc.py http://bugs.python.org/issue27614 opened by earl.chew #27618: docs for threading.Lock claim it's a class (since 3.3), but it http://bugs.python.org/issue27618 opened by gvanrossum #27619: getopt should strip whitespace from long arguments http://bugs.python.org/issue27619 opened by steven.daprano #27620: IDLE: Add keyboard equivalents for mouse actions. http://bugs.python.org/issue27620 opened by terry.reedy #27621: Finish IDLE Query dialog appearance and behavior. http://bugs.python.org/issue27621 opened by serhiy.storchaka #27623: int.to_bytes() and int.from_bytes(): raise ValueError when byt http://bugs.python.org/issue27623 opened by mmarkk #27627: clang fails to build ctypes on Android armv7 http://bugs.python.org/issue27627 opened by xdegaye #27628: ipaddress incompatibility with ipaddr: __contains__ between ne http://bugs.python.org/issue27628 opened by lukasz.langa #27629: Cannot create ssl.SSLSocket without existing socket http://bugs.python.org/issue27629 opened by nemunaire #27630: Generator._encoded_EMTPY misspelling in email package http://bugs.python.org/issue27630 opened by martin.panter #27632: build on AIX fails when builddir != srcdir, more than bad path http://bugs.python.org/issue27632 opened by Michael.Felt #27635: pickle documentation says that unpickling may not call __new__ http://bugs.python.org/issue27635 opened by july #27636: Refactor IDLE htest http://bugs.python.org/issue27636 opened by terry.reedy #27637: int.to_bytes(-1, ...) should automatically choose required cou http://bugs.python.org/issue27637 opened by mmarkk #27638: int.to_bytes() and int.from_bytes() should support 'net' and ' http://bugs.python.org/issue27638 opened by mmarkk #27639: UserList.__getitem__ doesn't account for slices http://bugs.python.org/issue27639 opened by staticshock #27640: add the '--disable-test-suite' option to configure http://bugs.python.org/issue27640 opened by xdegaye #27641: Do not build Programs/_freeze_importlib when cross-compiling http://bugs.python.org/issue27641 opened by thomas.perl #27643: test_ctypes fails on AIX with xlc http://bugs.python.org/issue27643 opened by Michael.Felt #27644: Expand documentation about type aliases and NewType in the typ http://bugs.python.org/issue27644 opened by michael0x2a #27645: Supporting native backup facility of SQLite http://bugs.python.org/issue27645 opened by lelit #27646: yield from expression can be any iterable http://bugs.python.org/issue27646 opened by ter
Re: [Python-Dev] PEP487: Simpler customization of class creation
Hi Sylvain,
thanks for the example, it's a great example to illustrate PEP 487 and
its design decisions.
> What it does is setting some class attributes that are required for certain
> types of descriptors to be able to initialize themselves.
>
> class MetaHasTraits(MetaHasDescriptors):
>
> """A metaclass for HasTraits."""
> def setup_class(cls, classdict):
> cls._trait_default_generators = {}
> super(MetaHasTraits, cls).setup_class(classdict)
While in a metaclass solution this does need that the metaclass needs
to execute code on the first class it is used for, in a PEP 487
solution this is not the case. A PEP 487 class HasTraits (no Meta
here) would not have Traits-descriptors itself, the classes inheriting
from it would have traits to be initialized. The PEP 487 HasTraits
takes the role of the metaclass.
A completely different problem shows up here. In your example,
HasTraits needs to initialize things on the class before the
Descriptors are run. This is not possible with PEP 487, where the
descriptors are initialized before __init_subclass__ is even called.
There are two ways to mitigate that problem:
- the first initialized descriptor could do the necessary initialization, or
- the descriptors are initialized from within __init_subclass__
At first sight, the first option seems hackish and the second option
looks much saner. Nonetheless PEP 487 proposes the second solution.
The rationale behind that is that people tend to forget to call
super(), and suddenly descriptors don't work anymore.
I realized later there is another benefit to this: if the first
initialized descriptor is doing the class initialization, often
__init_subclass__ isn't needed at all anymore, which means that those
kind of descriptors can be used on any class, without the need to tell
users that they have to inherit from a certain base class for the
descriptors to work. Only if some finalizing code needs to run after
the last descriptor is initialized one needs to write an
__init_subclass__. This is unavoidable as the last descriptor doesn't
know that it is the last.
Greetings
Martin
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[Python-Dev] Method signatures in the datetime module documentation
I have started [1] writing documentation for the new PEP 495 (Local Time Disambiguation) features and ran into the following problem. The current documentation is rather inconsistent in presenting the method signatures. For example: date.replace(year, month, day) [2], but datetime.replace([year[, month[, day[, hour[, minute[, second[, microsecond[, tzinfo) [3]. The new signature for datetime.replace in the Python implementation is def replace(self, hour=None, minute=None, second=None, microsecond=None, tzinfo=True, *, fold=None): but the C implementation does not accept True for tzinfo or None for the other arguments. The defaults in the Python implementation are really just sentinels to detect which arguments are not provided. How should I present the signature of the new replace method in the documentation? [1]: http://bugs.python.org/issue27595 [2]: https://docs.python.org/3/library/datetime.html#datetime.date.replace [3]: https://docs.python.org/3/library/datetime.html#datetime.datetime.replace ___ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP487: Simpler customization of class creation
On 07/29/2016 08:01 AM, Martin Teichmann wrote: ... Also, while researching other people's code when I wrote PEP 487, I couldn't find any such code elsewhere, yet I found a lot of code where people took the wildest measure to prevent a metaclass in doing its job on the first class it is used for. One example is enum.EnumMeta, which contains code not to make enum.Enum an enum ... Actually, enum.Enum is an enum. The guards are there because part of creating a new Enum class is searching for the previous Enum classes, and of course the very first time through there is no previous Enum class. My apologies if I have misunderstood what you were trying to say. -- ~Ethan~ ___ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
