how to dynamically instantiate an object inheriting from severalclasses?

I have a function that takes a reference to a class, and then
instantiates that class (and then does several other things with the
new instance). This is easy enough:

item = cls(self, **itemArgs)

where "cls" is the class reference, and itemArgs is obviously a set of
keyword arguments for its __init__ method.

But now I want to generalize this to handle a set of mix-in classes.
Normally you use mixins by creating a class that derives from two or
more other classes, and then instantiate that custom class. But in my
situation, I don't know ahead of time which mixins might be used and
in what combination. So I'd like to take a list of class references,
and instantiate an object that derives from all of them, dynamically.

Is this possible? If so, how?

Thanks,
- Joe

re: how to dynamically instantiate an object inheriting from severalclasses?

Joe Strout <joe@strout.netwrites:
Of course it's possible: use type(name, bases, dict).
........TrueTrue

Call-by-object'ly yours

--
Arnaud

re: how to dynamically instantiate an object inheriting from severalclasses?

On Nov 21, 5:11*pm, Joe Strout <j...@strout.netwrote:Easily:

derived_cls = type('Derived', (cls1, cls2, *rest_classes), {})
item = derived_cls(**itemArgs)

You will probably want to cache the generated classes so that at most
one class is created for each combination of mixins.

HTH,
George

re: how to dynamically instantiate an object inheriting from severalclasses?

On Nov 21, 2008, at 3:30 PM, Arnaud Delobelle wrote:
Thanks, I never knew about that form of type(). Neither does the
2.5.2 reference manual, whose only index entry for the type() function
is <http://www.python.org/doc/2.5.2/ref/objects.html#l2h-21>, and that
speaks only about the traditional use of type() to check the type of
an object.

help(type) does mention the form you show, though it doesn't explain
what the dict is for.

Where would I find documentation on this nifty function?

Thanks,
- Joe

re: how to dynamically instantiate an object inheriting from severalclasses?

In article <0E0333C1-9515-4736-A839-F59AAD675D4E@strout.net>,
Joe Strout <joe@strout.netwrote:Where built-in functions are documented, the Python Library Reference:

<http://www.python.org/doc/2.5.2/lib/built-in-funcs.html>

--
Ned Deily,
nad@acm.org

re: how to dynamically instantiate an object inheriting from severalclasses?

On Nov 21, 2008, at 6:06 PM, Ned Deily wrote:
Perfect, thank you. (Odd that the index entry for type() doesn't link
to this page.)

Best,
- Joe

re: how to dynamically instantiate an object inheriting from severalclasses?

Joe Strout wrote:Yeah, the indexing isn't perfect. Not sure what to do about that without
filing bugs for each case (which I am as guilty of not doing as anyone
else).

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/

re: how to dynamically instantiate an object inheriting from severalclasses?

On Fri, 21 Nov 2008 15:11:20 -0700, Joe Strout wrote:
Hmmm... how do you do that from Python code? The simplest way I can think
of is to extract the name of the class, and then pass the name as a
reference to the class, and hope it hasn't been renamed in the meantime:

def foo(cls_name, item_args):
# Won't necessarily work for nested scopes.
cls = globals()[cls_name]
item = cls(**itemArgs)
return item

instance = foo(Myclass.__name__, {'a':1})

Seems awfully complicated. If I were you, I'd forget the extra layer of
indirection and just pass the class itself, rather than trying to
generate some sort of reference to it. Let the Python virtual machine
worry about what is the most efficient mechanism to use behind the scenes.


[...]It sounds like you need to generate a new class on the fly. Here's one
way:

# untested
def foo(cls, item_args, mixins=None):
superclasses = [cls] + (mixins or [])
class MixedClass(*superclasses):
pass
item = MixedClass(**itemArgs)
return item

instance = foo(MyClass, {'a':1}, [Aclass, Bclass, Cclass])


--
Steven