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isinstance is broken

Michael Hudson mwh at python.net
Sat Jan 18 13:20:28 EST 2003 
mis6 at pitt.edu (Michele Simionato) writes:

> There are inconsistencies in "isinstance".
> 
> Example 1:
> 
> >>> isinstance(int,object)
> 1
> 
> gives no error message. I don't like it, an error should be
> raised.

Heck, no.  'int' is an instance of 'type'; 'type' is a subclass of
object.

> There is issubclass for that job:
> 
> >>> issubclass(int,object)
> 1

Asking a different question.

            [object]
             /    \
  (subclass)/      \(subclass)
           /        \
          /          \
       [int]=======[type]
           (instance)

> Example 2 
> 
> >>> class C: pass #does *not* inherit from object
> >>> c=C() 
> >>> isinstance(c,object) #???
> 1
> 
> It is clear that c is *not* an instance of class object: 

Yes it is: c is of type instance; instance is a subtype of object.

->> type(c)
<type 'instance'>
->> type(c).__bases__
(<type 'object'>,)

Here type(c) != c.__class__, one of the awkwardnesses that the
type/class unification removes.

> for example it doesn't have the __new__ attribute:
> 
> >>> c.__new__
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> AttributeError: C instance has no attribute '__new__'

However:

->> type(c).__new__
<built-in method __new__ of type object at 0x80f2380>

Objects/classobject.c:instance_getattr looks in c.__class__.

This function isn't much use, it seems.

> Moreover, on a related note:
> 
> >>> import re
> >>> reobj=re.compile('x')
> >>> isinstance(reobj,object)
> 1
> >>> reobj.__new__
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> AttributeError: __new__
> 
> but regular expression objects are *not* instances of object (this is
> also documented).

This is more or less the same as before.

> I wonder if there is a plan to make regular expression 
> objects regular objects in the near future

Not aware of any.

> For the moment, of course I can use
> 
> class RE(object):
>   def __init__(self,regexp):
>       reobj=re.compile(regexp) 
>       for attr in dir(reobj):
>           setattr(self,attr,getattr(reobj,attr))
> 
> REobj=RE('x')
> REobj.__new__
> <built-in method __new__ of type object at 0x80e6280>
> 
> Should I interpret the fact that isinstance(something,object) always returns
> true as a bug or is it intentional ?

Unavoidable would be my word.

Cheers,
M.

-- 
  ARTHUR:  Yes.  It was on display in the bottom of a locked filing
           cabinet stuck in a disused lavatory with a sign on the door
           saying "Beware of the Leopard".
                    -- The Hitch-Hikers Guide to the Galaxy, Episode 1
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