[Tutor] how to create a generic instance of an object?

[John Burk]

-----Original Message-----
From: Danny Yoo [mailto:dyoo at hkn.eecs.berkeley.edu] 
Sent: Tuesday, September 06, 2005 6:55 PM
To: John Burk
Cc: tutor at python.org
Subject: Re: [Tutor] how to create a generic instance of an object?

Hi John,

It sounds like you're trying to do some kind of dynamic linking.  Python
has a builtin called "__import__" that can help: it allows us to do
module
import, given an arbitrary name.

Your example above might be written as:

#####################################################################
<asset.py>
class Asset: pass

<foo.py>
class Foo(Asset): pass

<script.py>

def dynamic_lookup(module_name, class_name):
    module_object = __import__(module_name)
    class_object = getattr(module_object, class_name)
    return class_object

klass = dynamic_lookup('foo', 'Foo')
o = klass()
#####################################################################

This can be easily abused.  *grin* But for what you're trying, it might
be
what you're looking for.  For more information on __import__, see:

    http://www.python.org/doc/lib/built-in-funcs.html


Hope this helps!



And the winner is Danny Yoo, for the cleanest implementation.  Some of
my design goals for this class heirarchy were:

* end-users of the class heirarchy should be able to add new sub-classes
without modifying any of the other base or sub-classes.

* an exhaustive list of all sub-classes should be unneccesary

* likewise, any front-end script that uses this class shouldn't have a
long list of 'import' statements.

Danny's method helped me meet all these goals.  

The approach outlined above models the 'import <module>' behavior.  I'm
using a namespace to differentiate all our home-grown classes and
modules, and because I'm trying to model the 'from rad.<class> import
<Class>, my implementation looks like:

=============================================================
def dynamic_lookup(module_name, class_name):
    module_object = __import__(module_name, globals(), locals(),
[class_name])
    class_object = getattr(module_object, class_name)
    return class_object

aType = 'foo'
aName = 'bar'

klass = dynamic_lookup( 'rad.' + aType, aType.capitalize() )
o = klass(aName)
=============================================================

Thanks to all who replied.  

johnb