[Tutor] Using new style classes and __slots__

Kent Johnson kent37 at tds.net
Sat Sep 24 15:18:03 CEST 2005

mailing list wrote:
> Hi all,
> I'm just looking for a quick runthrough on the differences between new
> style classes and the old ones, and how to use the new ones.
> Also, how exactly do you use __slots__? I've got a 6000 object
> collection, and apparently using __slots__ will save memory, but only
> those attributes specified in __slots__ can be set, which suits my
> project fine.

All you have to do is define a __slots__ variable in your class. Its value is a list of attribute names. Then instances of your class will only be able to have the attributes named in __slots__:

 >>> class foo(object):
 ...   __slots__ = ['x', 'y']
 >>> f=foo()
 >>> f.x=1
 >>> f.y=2
 >>> f.z=3
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
AttributeError: 'foo' object has no attribute 'z'

Take a look at
http://www.python.org/2.2.3/descrintro.html (search for __slots__)

Note to the list: Use of __slots__ is generally discouraged, but IIUC this is exactly the use case it was designed for so I think it is OK.


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