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comp.lang.python

classobj() question

Roland Hedberg

2/12/2008 3:09:00 PM

Hi!

I'm in the position that I have a bunch of classes defined before hand
and then in some special circumstances I need to dynamically create a
class that has a number of the static classes as parents.

So I thought I could use classobj() from the new module, it seem exactly
what I wanted.

But, it doesn't perform as I expected.

I've made an extremely simple program to try to show what I mean and
what I expected. It's attached to this mail.

So, how should I have done it ?

-- Roland
2 Answers

Peter Otten

2/12/2008 4:20:00 PM

0

Roland Hedberg wrote:

> I'm in the position that I have a bunch of classes defined before hand
> and then in some special circumstances I need to dynamically create a
> class that has a number of the static classes as parents.
>
> So I thought I could use classobj() from the new module, it seem exactly
> what I wanted.
>
> But, it doesn't perform as I expected.
>
> I've made an extremely simple program to try to show what I mean and
> what I expected. It's attached to this mail.
>
> So, how should I have done it ?

(1) You are working with newstyle classes -- use type() instead of
new.classobj().
(2) Your inheritance tree has diamonds in it -- call baseclass initializers
using super() in *all* classes, e. g:

class A(object):
def __init__(self):
super(A, self).__init__()
# your code

Python will then take care that every initializer in the inheritance tree is
called once.
(3) You can explore the order in which the initializers are called by
printing the classes' __mro__ attribute for the final tweaks.

See

http://www.python.org/download/releases/2.2.2/descr...

for an introduction to newstyle multiple inheritance and

http://www.python.org/download/release...

if you want to dig deeper or like ascii art ;)

Peter

Roland Hedberg

2/12/2008 6:27:00 PM

0

Peter Otten wrote:
> Roland Hedberg wrote:
>
>> I'm in the position that I have a bunch of classes defined before hand
>> and then in some special circumstances I need to dynamically create a
>> class that has a number of the static classes as parents.
>>
>> So I thought I could use classobj() from the new module, it seem exactly
>> what I wanted.
>>
>> But, it doesn't perform as I expected.
>>
>> I've made an extremely simple program to try to show what I mean and
>> what I expected. It's attached to this mail.
>>
>> So, how should I have done it ?
>
> (1) You are working with newstyle classes -- use type() instead of
> new.classobj().
> (2) Your inheritance tree has diamonds in it -- call baseclass initializers
> using super() in *all* classes, e. g:
>
> class A(object):
> def __init__(self):
> super(A, self).__init__()
> # your code
>
> Python will then take care that every initializer in the inheritance tree is
> called once.

Thanks Peter, your hints was exactly what I needed. Now it works !

-- Roland