New-style classes (was Re: Python 2.2 properties)

Alex Martelli aleax at aleax.it
Sun Jan 20 17:20:04 CET 2002


Stefan Schwarzer wrote:

> I just wondered if there is something like
> 
> from __future__ import new_style_classes
> 
> to get the derivation from object by default if no base class is
> specified.

AFAIK, setting the module-global __metaclass__ is the way to 
do that.  However, not the way you do it:

>>> __metaclass__ = object

This doesn't work because object is not designed as a metaclass:

>>> __metaclass__ = object
>>> class X: pass
...
>>> x=X()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: object is not callable: <object object at 0x816eae0>
>>>

Rather:

>>> __metaclass__ = type
>>> class X: pass
...
>>> x=X()
>>>

type is the metaclass you want to use here (and you can set the
module-global __metaclass__ back to types.ClassType to go back
to having classic classes by default).


Alex




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