How to create objects dynamically in an elegant way in python?

Question

I have two classes that I would like to merge into a composite. These two classes will continue to be used standalone and I don't want to modify them. For some reasons, I want to let my composite class creating the objects. I am thinking about something like the code below (it is just an example) but I think it is complex and I don't like it very much. I guess that it could be improved by some techniques and tricks that I ignore.

Please note that the composite is designed to manage a lot of different classes with different constructor signatures.

What would recommend in order to improve this code?

class Parent:
    def __init__(self, x):
        self.x = x

class A(Parent):
    def __init__(self, x, a="a", b="b", c="c"):
        Parent.__init__(self, x)
        self.a, self.b, self.c = a, b, c

    def do(self):
        print self.x, self.a, self.b, self.c

class D(Parent):
    def __init__(self, x, d):
        Parent.__init__(self, x)
        self.d = d

    def do(self):
        print self.x, self.d

class Composite(Parent):
    def __init__(self, x, list_of_classes, list_of_args):
        Parent.__init__(self, x)
        self._objs = []
        for i in xrange(len(list_of_classes)):
            self._objs.append(self._make_object(list_of_classes[i], list_of_args[i]))

    def _make_object(self, the_class, the_args):
        if the_class is A:
            a = the_args[0] if len(the_args)>0 else "a"
            b = the_args[1] if len(the_args)>1 else "b"
            c = the_args[2] if len(the_args)>2 else "c"
            return the_class(self.x, a, b, c)
        if the_class is D:
            return the_class(self.x, the_args[0])

    def do(self):
        for o in self._objs: o.do()


compo = Composite("x", [A, D, A], [(), ("hello",), ("A", "B", "C")])
compo.do()

Are you talkning about Meta Classes? ibm.com/developerworks/linux/library/l-pymeta.html — Esteban Küber
– Esteban Küber, Commented Sep 2, 2009 at 13:57
Metaclasses are deeper magic than 99% of users should ever worry about. If you wonder whether you need them, you don't (the people who actually need them know with certainty that they need them, and don't need an explanation about why). -- Tim Peters — Esteban Küber
– Esteban Küber, Commented Sep 2, 2009 at 13:59
I don't know if metaclass can be a solution of this problem. I am not very familiar with this concept. I will look at the link you gave carefully. Thanks — luc
– luc, Commented Sep 2, 2009 at 14:06
I guess that my problem is more an 'object factory' issue while metaclass deals with 'class factory' issues. I am not sure about that and i am interested by any idea using such concept — luc
– luc, Commented Sep 2, 2009 at 14:12

Cat Plus Plus · Accepted Answer · 2009-09-02 14:52:47Z

2

You could shorten it by removing type-checking _make_object, and letting class constructors take care of the default arguments, e.g.

class Composite(Parent):
    def __init__(self, x, list_of_classes, list_of_args):
        Parent.__init__(self, x)
        self._objs = [
            the_class(self.x, *the_args)
            for the_class, the_args
            in zip(list_of_classes, list_of_args)
            if isinstance(the_class, Parent.__class__)
        ]

    def do(self):
        for o in self._objs: o.do()

This would also allow you to use it with new classes without modifying its code.

answered Sep 2, 2009 at 14:52

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1 Comment

luc Over a year ago

'if isinstance(the_class, Parent.__class__)' should be replaced by 'if issubclass(the_class, Parent)'. except this minor fix, it works great and corresponds to the kind of improvement I am looking for. Thanks

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