__bases__ misleading error message

Marco Buttu marco.buttu at gmail.com
Sat Jan 24 22:51:32 CET 2015

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
 >>> MyClass = Foo('MyClass', (), {})

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

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

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)
 >>> m.foo
Traceback (most recent call last):
AttributeError: 'MyClass' object has no attribute 'foo'

Marco Buttu

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