[Cython] [Python-Dev] C-level duck typing

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Mon May 28 17:59:43 CEST 2012

Dag Sverre Seljebotn <d.s.seljebotn at astro.uio.no> wrote:

>On 05/28/2012 01:24 PM, Nathaniel Smith wrote:
>> On Mon, May 28, 2012 at 12:09 PM, mark florisson
>> <markflorisson88 at gmail.com>  wrote:
>>> On 28 May 2012 12:01, Nathaniel Smith<njs at pobox.com>  wrote:
>>>> On Mon, May 28, 2012 at 11:55 AM, mark florisson
>>>> <markflorisson88 at gmail.com>  wrote:
>>>>> On 28 May 2012 11:41, Nathaniel Smith<njs at pobox.com>  wrote:
>>>>>> On Mon, May 28, 2012 at 10:13 AM, mark florisson
>>>>>> <markflorisson88 at gmail.com>  wrote:
>>>>>>> On 28 May 2012 09:54, mark florisson<markflorisson88 at gmail.com> 
>>>>>>>> On 27 May 2012 23:12, Nathaniel Smith<njs at pobox.com>  wrote:
>>>>>>>>> On Sun, May 27, 2012 at 10:24 PM, Dag Sverre Seljebotn
>>>>>>>>> <d.s.seljebotn at astro.uio.no>  wrote:
>>>>>>>>>> On 05/18/2012 10:30 AM, Dag Sverre Seljebotn wrote:
>>>>>>>>>>> On 05/18/2012 12:57 AM, Nick Coghlan wrote:
>>>>>>>>>>>> I think the main things we'd be looking for would be:
>>>>>>>>>>>> - a clear explanation of why a new metaclass is considered
>too complex a
>>>>>>>>>>>> solution
>>>>>>>>>>>> - what the implications are for classes that have nothing
>to do with the
>>>>>>>>>>>> SciPy/NumPy ecosystem
>>>>>>>>>>>> - how subclassing would behave (both at the class and
>metaclass level)
>>>>>>>>>>>> Yes, defining a new metaclass for fast signature exchange
>has its
>>>>>>>>>>>> challenges - but it means that *our* concerns about
>>>>>>>>>>>> consistent behaviour in the default object model and
>avoiding adverse
>>>>>>>>>>>> effects on code that doesn't need the new behaviour are
>>>>>>>>>>>> automatically.
>>>>>>>>>>>> Also, I'd consider a functioning reference implementation
>using a custom
>>>>>>>>>>>> metaclass a requirement before we considered modifying type
>anyway, so I
>>>>>>>>>>>> think that's the best thing to pursue next rather than a
>PEP. It also
>>>>>>>>>>>> has the virtue of letting you choose which Python versions
>to target and
>>>>>>>>>>>> iterating at a faster rate than CPython.
>>>>>>>>>>> This seems right on target. I could make a utility code C
>header for
>>>>>>>>>>> such a metaclass, and then the different libraries can all
>include it
>>>>>>>>>>> and handshake on which implementation becomes the real one
>>>>>>>>>>> sys.modules during module initialization. That way an
>eventual PEP will
>>>>>>>>>>> only be a natural incremental step to make things more
>polished, whether
>>>>>>>>>>> that happens by making such a metaclass part of the standard
>library or
>>>>>>>>>>> by extending PyTypeObject.
>>>>>>>>>> So I finally got around to implementing this:
>>>>>>>>>> https://github.com/dagss/pyextensibletype
>>>>>>>>>> Documentation now in a draft in the NumFOCUS SEP repo, which
>I believe is a
>>>>>>>>>> better place to store cross-project standards like this. (The
>>>>>>>>>> docstring standard will be SEP 100).
>>>>>>>>>> https://github.com/numfocus/sep/blob/master/sep200.rst
>>>>>>>>>> Summary:
>>>>>>>>>>   - No common runtime dependency
>>>>>>>>>>   - 1 ns overhead per lookup (that's for the custom slot
>*alone*, no
>>>>>>>>>> fast-callable signature matching or similar)
>>>>>>>>>>   - Slight annoyance: Types that want to use the metaclass
>must be a
>>>>>>>>>> PyHeapExtensibleType, to make the binary layout work with how
>CPython makes
>>>>>>>>>> subclasses from Python scripts
>>>>>>>>>> My conclusion: I think the metaclass approach should work
>really well.
>>>>>>>>> Few quick comments on skimming the code:
>>>>>>>>> The complicated nested #ifdef for __builtin_expect could be
>simplified to
>>>>>>>>>   #if defined(__GNUC__)&&  (__GNUC__>  2 || __GNUC_MINOR__> 
>>>>>>>>> PyCustomSlots_Check should be called PyCustomSlots_CheckExact,
>>>>>>>>> And given that, how can this code work if someone does
>subclass this
>>>>>>>>> metaclass?
>>>>>>>> I think we should provide a wrapper for PyType_Ready, which
>>>>>>>> copies the pointer to the table and the count directly into the
>>>>>>>> subclass. If a user then wishes to add stuff, the user can
>allocate a
>>>>>>>> new memory region dynamically, memcpy the base class' stuff in
>>>>>>>> and append some entries.
>>>>>>> Maybe we should also allow each custom type to set a
>>>>>>> since they are then heap types which can go out of scope. The
>>>>>>> metaclass can then call this deallocator to deallocate the
>>>>>> Custom types are plain old Python objects, they can use
>>>>> If I set etp_custom_slots to something allocated on the heap, then
>>>>> (shared) metaclass would have to deallocate it. The tp_dealloc of
>>>>> type itself would be called for its instances (which can be used
>>>>> deallocate dynamically allocated memory in the objects if you use
>>>>> custom slot "pointer offset").
>>>> Oh, I see. Right, the natural way to handle this would be have each
>>>> user define their own metaclass with the behavior they want.
>>>> argument for supporting multiple metaclasses simultaneously I
>>>> - N
>>>> _______________________________________________
>>>> cython-devel mailing list
>>>> cython-devel at python.org
>>>> http://mail.python.org/mailman/listinfo/cython-devel
>>> That bludgeons your constant time type check.
>> Not if you steal a flag, like the interpreter already does with
>> referring to that argument I made earlier :-)
>>> It's easier to just
>>> reserve an extra slot for a deallocator pointer :) It would probably
>>> be set to NULL in the common case anyway, since you allocate your
>>> slots statically.
>Subclassing: Note that even if all types has to have a PyHeapTypeObject
>structure, they are still statically allocated! So for statically 
>created subclasses (which should be the majority of the cases), there's
>not going to be any deallocator...
>I agree that there should be a PyExtensibleType_Ready. To keep 
>allocating statically I propose that the subclass should leave some
>open for slots from the superclass:
>PyCustomSlot subclass_custom_slots[10] = {
>   {SLOT_C, foo}, {SLOT_D, BAR}, {0,0}, ...
>Then, fill in etp_count=2, etp_custom_slots=subclass_custom_slots, and 
>then call PyExtensibleType_Ready(&Subclass_Type, 10); i.e., the number 
>of total elements in etp_custom_slots is passed in.
>One should always leave more room than one thinks one needs if the 
>superclass is from another library...
>Then, inheritance happens according to the following rules:
>  - Slots are inherited from superclass
>  - Slots in subclass with same ID overwrites superclass
>  - Slots from superclass are put before slots from subclass
>  - Exception raised if the number of final slots is larger than the 
>limit passed in to PyExtensibleType_Ready.
>(Whenever this is not sufficient, you can always manually munge the 
>table after PyExtensibleType_Ready.)
>Question: How to deal with possible flag bits in the ID?
>Three approaches:
>a) Forget about the flags-in-ID idea; if you want flags, stick them in 
>the data
>  b) Embed a seperate variable for flags in every PyCustomSlot
>  c) Standardize on a *hard* requirement on the bottom 8 bits being 
>flags while the top 24 bits indicate incompatible slots; so for the 
>purposes of inheritance, 0x12345601 would overwrite 0x12345600.
>To me, b) is OK, but the 32 bit ID space is already so ridiculously
>that c) is a "why not"? -1 on a), it'd be rather tedious if the payload

I guess the sane thing to do is make the custom slot (id, flags, data); and have id and flags be 32 bits on all platforms. Otherwise 32 bits are wasted to padding on 64 bit platforms anyway.

Is there a type one can safely use everywhere to get a 32 bit unsigned int? Does MSVC support stdint.h?


>is an offset to the PyObject*.
>Subclassing in heap-allocated types (subclasses Python side): It'd 
>certainly be nice to completely ignore this for now and require making
>sub-metaclass to support this (e.g., have tp_new parse some 
>__customslots__ attribute in the class dict).
>Hijacking a TP_FLAG: We could make the branch for a direct hit on 
>metaclass comparison likely(), so that the branch checking tp_base on 
>the metaclass unlikely(), which with branch prediction I think makes it
>very likely that there's no penalty for allowing sub-metaclasses (when 
>you don't use them -- when you do, there's a slight penalty).
>But here's another great argument in favour of a TP_FLAG bit: Consumers
>would then not need to import the metaclass or contain its definition 
>(which is really only around in case the user imports the consumer 
>before the provider...). This would make the header file the consumers 
>need to bundle much lighter. So I think I'm +1.
>At any rate, I would like the metaclass rendezvous to keep happening 
>just because it's less confusing if "extensibletype is extensibletype" 
>in general.
>Anyway, the metaclass checking is a nice fallback if CPython uses all 
>their flag bits.
>cython-devel mailing list
>cython-devel at python.org

Sent from my Android phone with K-9 Mail. Please excuse my brevity.

More information about the cython-devel mailing list