[Python-Dev] Python 3, new-style classes and __class__

[Michael Foord]

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
>
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