[Python-Dev] Python 3, new-style classes and __class__
Michael Foord
fuzzyman at voidspace.org.uk
Mon Nov 21 18:22:48 CET 2011
On 20/11/2011 21:41, Guido van Rossum wrote:
> On Sun, Nov 20, 2011 at 10:44 AM, Michael Foord
> <fuzzyman at voidspace.org.uk> wrote:
>> On 20 Nov 2011, at 16:35, Guido van Rossum wrote:
>>
>>> Um, what?! __class__ *already* has a special meaning. Those examples
>>> violate that meaning. No wonder they get garbage results.
>>>
>>> The correct way to override isinstance is explained here:
>>> http://www.python.org/dev/peps/pep-3119/#overloading-isinstance-and-issubclass
>>> .
>>>
>>
>> Proxy classes have been using __class__ as a descriptor for this purpose for years before ABCs were introduced. This worked fine up until Python 3 where the compiler magic broke it when super is used. That is now fixed anyway.
> Hm, okay. Though it's disheartening that it took three releases of 3.x
> to figure this out. And there was a PEP even!
>
>> If I understand correctly, ABCs are great for allowing classes of objects to pass isinstance checks (etc) - what proxy, lazy and mock objects need is to be able to allow individual instances to pass different isinstance checks.
> Ah, oops. Yes, __instancecheck__ is for the class to override
> isinstance(inst, cls); for the *instance* to override apparently
> you'll need to mess with __class__.
>
> I guess my request at this point would be to replace '@__class__' with
> some other *legal* __identifier__ that doesn't clash with existing use
> -- I don't like the arbitrary use of @ here.
>
The problem with using a valid identifier name is that it leaves open
the possibility of the same "broken" behaviour (removing from the class
namespace) for whatever name we pick.
That means we should document the name used - and it's then more likely
that users will start to rely on this odd (but documented) internal
implementation detail. This in turn puts a burden on other
implementations to use the same mechanism, even if this is less than
ideal for them.
This is why a deliberately invalid identifier was picked.
All the best,
Michael Foord
> --Guido
>
>> All the best,
>>
>> Michael Foord
>>
>>> --Guido
>>>
>>> On Sat, Nov 19, 2011 at 6:13 PM, Michael Foord
>>> <fuzzyman at voidspace.org.uk> wrote:
>>>>
>>>> On 19 November 2011 23:11, Vinay Sajip<vinay_sajip at yahoo.co.uk> wrote:
>>>>> Michael Foord<fuzzyman<at> voidspace.org.uk> writes:
>>>>>
>>>>>> That works fine in Python 3 (mock.Mock does it):
>>>>>>
>>>>>> >>> class Foo(object):
>>>>>> ... @property
>>>>>> ... def __class__(self):
>>>>>> ... return int
>>>>>> ...
>>>>>> >>> a = Foo()
>>>>>> >>> isinstance(a, int)
>>>>>> True
>>>>>> >>> a.__class__
>>>>>> <class 'int'>
>>>>>>
>>>>>> There must be something else going on here.
>>>>>>
>>>>> Michael, thanks for the quick response. Okay, I'll dig in a bit further:
>>>>> the
>>>>> definition in SimpleLazyObject is
>>>>>
>>>>> __class__ = property(new_method_proxy(operator.attrgetter("__class__")))
>>>>>
>>>>> so perhaps the problem is something related to the specifics of the
>>>>> definition.
>>>>> Here's what I found in initial exploration:
>>>>>
>>>>> --------------------------------------------------------------------------
>>>>> Python 2.7.2+ (default, Oct 4 2011, 20:06:09)
>>>>> [GCC 4.6.1] on linux2
>>>>> Type "help", "copyright", "credits" or "license" for more information.
>>>>>>>> from django.utils.functional import SimpleLazyObject
>>>>>>>> fake_bool = SimpleLazyObject(lambda: True)
>>>>>>>> fake_bool.__class__
>>>>> <type 'bool'>
>>>>>>>> fake_bool.__dict__
>>>>> {'_setupfunc':<function<lambda> at 0xca9ed8>, '_wrapped': True}
>>>>>>>> SimpleLazyObject.__dict__
>>>>> dict_proxy({
>>>>> '__module__': 'django.utils.functional',
>>>>> '__nonzero__':<function inner at 0xca9de8>,
>>>>> '__deepcopy__':<function __deepcopy__ at 0xca9c08>,
>>>>> '__str__':<function inner at 0xca9b18>,
>>>>> '_setup':<function _setup at 0xca9aa0>,
>>>>> '__class__':<property object at 0xca5730>,
>>>>> '__hash__':<function inner at 0xca9d70>,
>>>>> '__unicode__':<function inner at 0xca9b90>,
>>>>> '__bool__':<function inner at 0xca9de8>,
>>>>> '__eq__':<function inner at 0xca9cf8>,
>>>>> '__doc__': '\n A lazy object initialised from any function.\n\n
>>>>> Designed for compound objects of unknown type. For builtins or
>>>>> objects of\n known type, use django.utils.functional.lazy.\n ',
>>>>> '__init__':<function __init__ at 0xca9a28>
>>>>> })
>>>>> --------------------------------------------------------------------------
>>>>> Python 3.2.2 (default, Sep 5 2011, 21:17:14)
>>>>> [GCC 4.6.1] on linux2
>>>>> Type "help", "copyright", "credits" or "license" for more information.
>>>>>>>> from django.utils.functional import SimpleLazyObject
>>>>>>>> fake_bool = SimpleLazyObject(lambda : True)
>>>>>>>> fake_bool.__class__
>>>>> <class 'django.utils.functional.SimpleLazyObject'>
>>>>>>>> fake_bool.__dict__
>>>>> {
>>>>> '_setupfunc':<function<lambda> at 0x1c36ea8>,
>>>>> '_wrapped':<object object at 0x1d88b70>
>>>>> }
>>>>>>>> SimpleLazyObject.__dict__
>>>>> dict_proxy({
>>>>> '__module__': 'django.utils.functional',
>>>>> '__nonzero__':<function inner at 0x1f56490>,
>>>>> '__deepcopy__':<function __deepcopy__ at 0x1f562f8>,
>>>>> '__str__':<function inner at 0x1f561e8>,
>>>>> '_setup':<function _setup at 0x1f56160>,
>>>>> '__hash__':<function inner at 0x1f56408>,
>>>>> '__unicode__':<function inner at 0x1f56270>,
>>>>> '__bool__':<function inner at 0x1f56490>,
>>>>> '__eq__':<function inner at 0x1f56380>,
>>>>> '__doc__': '\n A lazy object initialised from any function.\n\n
>>>>> Designed for compound objects of unknown type. For builtins or
>>>>> objects of\n known type, use django.utils.functional.lazy.\n ',
>>>>> '__init__':<function __init__ at 0x1f560d8>
>>>>> })
>>>>> --------------------------------------------------------------------------
>>>>>
>>>>> In Python 3, there's no __class__ property as there is in Python 2,
>>>>> the fake_bool's type isn't bool, and the callable to set up the wrapped
>>>>> object never gets called (which is why _wrapped is not set to True, but to
>>>>> an anonymous object - this is set in SimpleLazyObject.__init__).
>>>>>
>>>> The Python compiler can do strange things with assignment to __class__ in
>>>> the presence of super. This issue has now been fixed, but it may be what is
>>>> biting you:
>>>>
>>>> http://bugs.python.org/issue12370
>>>>
>>>> If this *is* the problem, then see the workaround suggested in the issue.
>>>> (alias super to _super in the module scope and use the old style super
>>>> calling convention.)
>>>>
>>>> Michael
>>>>
>>>>
>>>>> Puzzling!
>>>>>
>>>>> Regards,
>>>>>
>>>>> Vinay Sajip
>>>>>
>>>>> _______________________________________________
>>>>> Python-Dev mailing list
>>>>> Python-Dev at python.org
>>>>> http://mail.python.org/mailman/listinfo/python-dev
>>>>> Unsubscribe:
>>>>> http://mail.python.org/mailman/options/python-dev/fuzzyman%40voidspace.org.uk
>>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> http://www.voidspace.org.uk/
>>>>
>>>> May you do good and not evil
>>>> May you find forgiveness for yourself and forgive others
>>>>
>>>> May you share freely, never taking more than you give.
>>>> -- the sqlite blessing http://www.sqlite.org/different.html
>>>>
>>>> _______________________________________________
>>>> Python-Dev mailing list
>>>> Python-Dev at python.org
>>>> http://mail.python.org/mailman/listinfo/python-dev
>>>> Unsubscribe:
>>>> http://mail.python.org/mailman/options/python-dev/guido%40python.org
>>>>
>>>>
>>>
>>>
>>> --
>>> --Guido van Rossum (python.org/~guido)
>>>
>>
>> --
>> http://www.voidspace.org.uk/
>>
>>
>> May you do good and not evil
>> May you find forgiveness for yourself and forgive others
>> May you share freely, never taking more than you give.
>> -- the sqlite blessing
>> http://www.sqlite.org/different.html
>>
>>
>>
>>
>>
>>
>
>
--
http://www.voidspace.org.uk/
May you do good and not evil
May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
-- the sqlite blessing http://www.sqlite.org/different.html
More information about the Python-Dev
mailing list