Re: bases misleading error message

Date	2015-01-24 22:51 +0100
From	Marco Buttu <marco.buttu@gmail.com>
Newsgroups	comp.lang.python
Subject	Re: __bases__ misleading error message
References	<1a194e0a0b738d205de54180fa7@nntp.aioe.org> <mailman.18093.1422127493.18130.python-list@python.org>
Message-ID	<mailman.18100.1422136299.18130.python-list@python.org> (permalink)

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On 24/01/2015 20:24, Terry Reedy wrote:

> On 1/24/2015 5:16 AM, Mario Figueiredo wrote:
>>
>> Consider the following code at your REPL of choice
>
>>         class Sub:
>>             pass
>>
>>         foo = Sub()
>>
>>         Sub.__bases__
>>         foo.__bases__
>>
>> The last statement originates the following error:
>
> This is an anomalous situation.  Normally, if a class has an attribute,
> instances have the same attribute (unless overriden).  But this does not
> matter.

That is not true: if a class has an attribute, you can not say its 
instances have the same attribute. You can just say if a type defines an 
attribute, all its instances have that attribute. Look at this example:

 >>> class Foo(type):
...     foo = 33
...
 >>> Foo.foo
33
 >>> MyClass = Foo('MyClass', (), {})

MyClass is an instance of Foo, so it must have the attribute foo:

 >>> isinstance(MyClass, Foo)
True
 >>> MyClass.foo
33

But an instance of MyClass is not an instance of Foo, and so MyClass() 
must not have the attribute foo. In fact:

 >>> m = MyClass()
 >>> isinstance(m, Foo)
False
 >>> m.foo
Traceback (most recent call last):
     ...
AttributeError: 'MyClass' object has no attribute 'foo'

-- 
Marco Buttu

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__bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 10:16 +0000
  Re: __bases__ misleading error message Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-24 23:43 +1100
    Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 22:14 +0100
      Re: __bases__ misleading error message Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-24 14:45 -0700
        Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 23:09 +0100
          Re: __bases__ misleading error message Chris Angelico <rosuav@gmail.com> - 2015-01-25 09:25 +1100
            Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 23:33 +0100
              Re: __bases__ misleading error message Chris Angelico <rosuav@gmail.com> - 2015-01-25 09:37 +1100
                Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 23:59 +0100
      Re: __bases__ misleading error message Terry Reedy <tjreedy@udel.edu> - 2015-01-24 16:58 -0500
        Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 23:02 +0100
          Re: __bases__ misleading error message Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-24 15:16 -0700
            Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 23:36 +0100
      Re: __bases__ misleading error message Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-25 14:18 +1100
        Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-25 12:07 +0100
          Re: __bases__ misleading error message Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-25 23:00 +1100
            Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-25 13:49 +0100
              Re: __bases__ misleading error message Marko Rauhamaa <marko@pacujo.net> - 2015-01-25 14:53 +0200
            Re: __bases__ misleading error message Terry Reedy <tjreedy@udel.edu> - 2015-01-25 16:35 -0500
            Re: __bases__ misleading error message Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-25 19:21 -0700
    Re: __bases__ misleading error message Marco Buttu <marco.buttu@gmail.com> - 2015-01-24 23:09 +0100
    Re: __bases__ misleading error message Marco Buttu <marco.buttu@gmail.com> - 2015-01-24 15:12 +0100
  Re: __bases__ misleading error message Terry Reedy <tjreedy@udel.edu> - 2015-01-24 14:24 -0500
    Re: __bases__ misleading error message Mario Figueiredo <marfig@gmail.com> - 2015-01-24 22:03 +0100
    Re: __bases__ misleading error message Marco Buttu <marco.buttu@gmail.com> - 2015-01-24 22:51 +0100
      Re: __bases__ misleading error message Terry Reedy <tjreedy@udel.edu> - 2015-01-24 19:55 -0500
        Re: __bases__ misleading error message Marco Buttu <marco.buttu@gmail.com> - 2015-01-25 11:30 +0100
    Re: __bases__ misleading error message Marco Buttu <marco.buttu@gmail.com> - 2015-01-24 22:51 +0100

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