how to create instances of classes without calling the constructor?

[Steven D'Aprano]

On Tue, 18 Mar 2008 01:08:45 +0100, Dominik Jain wrote:

> Hi!
> 
> Does anyone know how an instance of a (new-style) class can be created
> without having to call the constructor (and providing the arguments it
> requires)? With old-style classes, this was possible using new.instance.
> Surely there must be a simple way to do this with new-style classes, too
> -- or else how does pickle manage?

I don't think you can create an instance without calling __new__. (At 
least not without all sorts of metaclass tricks.) Creating the instance 
is what __new__ does.

But you can create a new instance without calling the initialiser 
__init__. Here's a variation on a recipe from Alex Martelli:


class Spam(object):
    def __init__(self, *args):
        print "spam spam spam spam"
        self.args = args


def empty_instance(cls):
    class Empty(cls):
        def __init__(self): pass
    x = Empty()
    x.__class__ = cls
    return x



>>> # Create a Spam instance the regular way
... a = Spam("args")
spam spam spam spam
>>> # And a Spam instance the unusual way
... b = empty_instance(Spam)
>>> a.args
('args',)
>>> b.args
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Spam' object has no attribute 'args'



-- 
Steven