Code design problem

[Sion Arrowsmith]

Marco Nawijn  <nawijn at gmail.com> wrote:
>I have a hard time figuring out an elegant and efficient design for
>the following problem.

What you want is known as the factory pattern.

> [ ... ] I would
>like the following (pseudo)-code to work:
>
>app = Application('patran')                  # Run on local machine
>app.start(args)
>
>app = Application('patran', host='myhost')   # Run on remote machine
>app.start(args)
>
>The Application, local implementation and remote implementation all
>have the same interface, so a possibility might be something like the
>following:
>
>class Interface(object):
>     .....
>     def start(self): pass
>     def stop(self): pass

You don't need an abstract interface like this in Python. Duck-
typing will take care of it for you. On the other hand, if you
do have common code between the local and global implementations,
it makes sense to share it in a base class. You might then want
to consider defining the methods which must be overridden as

def start(self): raise NotImplementedError
etc.

Given:

>class LocalImplementation(Interface):
>       .....
>
>class GlobalImplementation(CorbaGlobalImplementation, Interface):
>       .....

with suitable start and stop methods defined, all you need is
an Application factory function:

def Application(program, host=None):
    if host is None:
        return LocalImplementation(program)
    return GlobalImplementation(program, host)

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