Can you create an instance of a subclass with an existing instance of the base class?

[Sandra-24]

Can you create an instance of a subclass using an existing instance of
the base class?

Such things would be impossible in some languages or very difficult in
others. I wonder if this can be done in python, without copying the
base class instance, which in my case is a very expensive object.

Any ideas?

Thanks,
-Sandra

[Lawrence D'Oliveiro]

In article <11************ **********@j33g 2000cwa.googleg roups.com>,
"Sandra-24" <sa***********@ yahoo.com> wrote:

Can you create an instance of a subclass using an existing instance of
the base class?

I think you're taking Python's OO-ness too seriously. One of the
strengths of Python is that it can _look_ like an OO language without
actually being OO.

[Cavingdeep]

With new-style classes you can find out a class' subclasses and then
you can instantiate the subclass you want. Suppose you have two classes
A and B, B is a subclass of A, A is a new-style class. Now you have an
A's instance called "a", to instance B you can do the following:

b = a.__class__.__s ubclasses__()[0]()

[Peter Otten]

Sandra-24 wrote:

Can you create an instance of a subclass using an existing instance of
the base class?

Such things would be impossible in some languages or very difficult in
others. I wonder if this can be done in python, without copying the
base class instance, which in my case is a very expensive object.

You can change the class of an instance by assigning to the __class__
attribute. The new class doesn't even need to be a subclass of the old:

class A(object): .... def __init__(self, name):
.... self.name = name
.... def show(self): print self.name
.... a = A("alpha")
a.show() alpha class B(object): .... def show(self): print self.name.upper ()
.... a.__class__ = B
a.show()

ALPHA

Peter

[Sandra-24]

Now that is a clever little trick. I never would have guessed you can
assign to __class__, Python always surprises me in it's sheer
flexibility.

In this case it doesn't work.

TypeError: __class__ assignment: only for heap types

I suspect that's because this object begins its life in C code.

The technique of using the __class__.__sub classes__ also fails:

TypeError: cannot create 'B' instances

This seems more complex than I thought. Can one do this for an object
that beings it's life in C?

Thanks,
-Sandra

Peter Otten wrote:

Sandra-24 wrote:

Can you create an instance of a subclass using an existing instance of
the base class?

Such things would be impossible in some languages or very difficult in
others. I wonder if this can be done in python, without copying the
base class instance, which in my case is a very expensive object.

You can change the class of an instance by assigning to the __class__
attribute. The new class doesn't even need to be a subclass of the old:

class A(object): ... def __init__(self, name):
... self.name = name
... def show(self): print self.name
... a = A("alpha")
a.show() alpha class B(object): ... def show(self): print self.name.upper ()
... a.__class__ = B
a.show()

ALPHA

Peter

[Lawrence D'Oliveiro]

In article <11************ *********@v46g2 000cwv.googlegr oups.com>,
"Sandra-24" <sa***********@ yahoo.com> wrote:

Now that is a clever little trick. I never would have guessed you can
assign to __class__, Python always surprises me in it's sheer
flexibility.

That's because you're still thinking in OO terms.

[Peter Otten]

Sandra-24 wrote:

Now that is a clever little trick. I never would have guessed you can
assign to __class__, Python always surprises me in it's sheer
flexibility.

In this case it doesn't work.

TypeError: __class__ assignment: only for heap types

I suspect that's because this object begins its life in C code.

The technique of using the __class__.__sub classes__ also fails:

TypeError: cannot create 'B' instances

This seems more complex than I thought. Can one do this for an object
that beings it's life in C?

The restriction originates in the metaclass. Perhaps you can use a
customized metaclass that allows subclass assignment, but I don't know
enough about Python's internals to tell you how/whether that is possible.

An alternative might be that you always start out with a subclass coded in
Python:

abc = tuple("abc")
abc.__class__ = tuple Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: __class__ assignment: only for heap types

class Tuple(tuple): pass .... class Subclass(Tuple) : .... first = property(lambda self: self[0])
.... abc = Tuple("abc")
abc.__class__ = Subclass
abc.first

'a'

In the example a Tuple should be able to do everything a tuple can do;
because Tuple is coded in Python you can also change the __class__.

Peter

[Sandra-24]

Lawrence D'Oliveiro wrote:

In article <11************ *********@v46g2 000cwv.googlegr oups.com>,
"Sandra-24" <sa***********@ yahoo.com> wrote:

Now that is a clever little trick. I never would have guessed you can
assign to __class__, Python always surprises me in it's sheer
flexibility.

That's because you're still thinking in OO terms.

It's not quite as simple as all that. I agree that people, escpecially
people with a Java (ew) background overuse OO, when there's often
simpler ways of doing things.

However in this case I'm simply getting an object (an mp_request object
from mod_python) passed into my function, and before I pass it on to
the functions that make up and individual web page it is modified by
adding members and methods to add functionality. It's not that I'm
thinking in OO, but that the object is a convienient place to put
things, especially functions that take an mp_request object as their
first argument.

Sadly I'm unable to create it as a python object first, because it's
created by the time my code comes into play. So I have to resort to
using the new module to add methods.

It works, but it has to be redone for every request, I thought moving
the extra functionality to another object would simplify the task. A
better way might be to contain the mp_request within another object and
use __getattr__ to lazily copy the inner object. I'd probably have to
first copy those few fields that are not read-only or use __setattr__
as well.

Thanks,
-Sandra

[Lawrence D'Oliveiro]

In article <11************ *********@v46g2 000cwv.googlegr oups.com>,
"Sandra-24" <sa***********@ yahoo.com> wrote:

However in this case I'm simply getting an object (an mp_request object
from mod_python) passed into my function, and before I pass it on to
the functions that make up and individual web page it is modified by
adding members and methods to add functionality.

All you want is a dictionary, then. That's basically what Python objects
are.

[bruno at modulix]

Lawrence D'Oliveiro wrote:
(snip)

I think you're taking Python's OO-ness too seriously. One of the
strengths of Python is that it can _look_ like an OO language without
actually being OO.

According to which definition of OO ?

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom. gro'.split('@')])"