Dynamic creation of an object instance of a class by name

[Jacek Generowicz]

meyer at acm.org (Andre Meyer) writes:

> Hi all
> 
> I have been searching everywhere for this, but have not found a
> solution, yet.
> 
> What I need is to create an object that is an instance of a class (NOT a
> class instance!)

I'm sorry, I can't for the life of me figure out what the (semantic)
difference is between "intsance of a class" and "class instance".

> of which I only know the name as a string. This what I
> tried:
> 
> class A:
>     def __init__(self, id):
>        self.id = id
>     def getID(self):
>       print self.id
> 
> ca = new.classobj('A', (), {})

This creates a new class which will be called "A", displacing your
original class by the same name.

> oa1 = ca()
> oa2 = new.instance(ca)
> 
> oa1
> <__main__.A instance at 0x007E8AF8>
> 
> oa1.getID()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
> AttributeError: A instance has no attribute 'getID'
> 
> oa2
> <__main__.A instance at 0x007E8AF8>
> 
> oa2.getID()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
> AttributeError: A instance has no attribute 'getID'
> 
> 
> Thus, both ways only produce a class instance, but NOT an object of that
> class!!!

Yes, they produce an instance of your newly created class.

> How can I get a new object instance???

I think you mean to ask "How can I get a new instance of the class
whose name I have in a string?". In other words, you should be asking
yourself, "How can I get my hands on an object whose name[*] I have in a
string?" (One possible approach is shown below.)

> What else needs to be filled in for base_classes and __dict__ in
> new.classobj?

You can't do it that way ... that way you create a _new_ separate
class called "A".

Here's how you might achieve what you want:

>>> class A:
...     def __init__(self, id):
...        self.id = id
...     def getID(self):
...       print self.id
... 
>>> oa1 = globals()['A'](12345)
>>> oa1
<__main__.A instance at 0x8190844>
>>> oa1.getID()
12345
>>> 


[*] I'm a bit wary about using "name" here, because I don't really
    mean the name of the class, but the name of a variable that
    happens to refer to the class.
    
    For example, consider (in the context established above)
    
    >>> B = A
    >>> del A
    >>> B.__name__
    'A'
    >>> A.__name__
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    NameError: name 'A' is not defined
    
    So, while the name of the class is still "A", it is no longer
    known by an identifier of that name.