Named integers and enums

[Steve Holden]

Hallvard B Furuseth wrote:

> Scott David Daniels wrote:
> 
>>Hallvard B Furuseth wrote:
>>
>>
>>>I wonder: isinstance() says that even classic class instances are 'object'
>>>instances, even though they do not seem to be (since __slots__ does not
>>>work):
>>>  >>> class o: __slots__ = ()
>>>  ... 
>>>  >>> isinstance(o(), object)
>>>  1
>>>  >>> o().foo = True
>>>  >>> 
>>
>>Actually __slots__ works, you just have a funny idea of what __slots__
>>is _for_.  __slots__ is a _storage_optimization_.
> 
> 
> That's not what I mean.
> 
> I'm wondering why isinstance() claims that instances of 'o' are
> instances of 'object', when 'o' is not declared to be a subclass of
> 'object'.
> 
Instances of o are instances of object because, I believe, everything is 
ultimately held to be an instance of object. Unless you can find an x 
such that

	isinstance(x, object)

is false - I couldn't find such a value.

> I used __slots__ to check if 'o' is a subclass of 'object' anyway, since
> __slots__ is documented to only work for new-style classes - i.e.
> classes derived from 'object'.  Well, it is not.  If it had been, the
> assignment to o().foo above would have failed.
> 
As you appear to have deduced, the purpose of __slots__ is to limit the 
keys that can be stored in the object's __dict__, and so clearly o is 
not a subclass of object. But you could have determined that directly:

 >>> class o: pass
...
 >>> issubclass(o, object)
False
 >>> issubclass(int, object)
True

However, none of this tells you anything about the relationship between 
object, old-style class and instances thereof:

 >>> type(o)
<type 'classobj'>
 >>> oi = O()
 >>> type(oi)
<type 'instance'>
 >>> issubclass(oi, object)
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
TypeError: issubclass() arg 1 must be a class

Sometimes it's difficult to keep your head from exploding:

 >>> type(object)
<type 'type'>
 >>> type(type)
<type 'type'>

regards
  Steve