Code design problem

Bruno Desthuilliers bruno.42.desthuilliers at
Wed Aug 29 15:16:54 CEST 2007

Marco Nawijn a écrit :
> Hello,
> I have a hard time figuring out an elegant and efficient design for
> the following problem.
> I am working on automation of structural design problems. In the
> majority of cases, this boils down to executing programs in batch in
> one or more loops. The scripts to control the execution differ from
> fortran to bash to python and so on. Most of them are ad hoc and what
> I call 'throw away scripts'. In order to improve the situation I would
> like to develop a Python module that supports the execution of
> external programs. Ideally I would like to make running locally or
> remote trivial for the users of the module. As an example, 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 problem I face is that the implementation of the application class
> is completely
> different for the local and remote case. The local case is a
> straightforward implemenation using the subprocess module, the remote
> case is a CORBA implementation. Somehow I would like to switch from
> implementation class at runtime depending on whether or not the host
> parameter is specified or not.

The solution is quite straightforward, and is known as the "factory" 
design pattern.

> 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

What's the use of this class ? In Python, inheritance is for 
implementation only.

> class LocalImplementation(Interface):
>        .....
> class GlobalImplementation(CorbaGlobalImplementation, Interface):
>        .....
> class Application(Interface):
>       def __init__(self, program, host=None):
>             ....
>             if host:
>                    self.__impl = LocalImplementation(program)
>             else:
>                    self.__impl = GlobalImplementation(program, host)
>       #  Forward all methods to the implementation class
>       def start(self):
>           self.__impl.start()
>       def stop(self):
>           self.__impl.stop()

My my my... How to uselessly overcomplexify things...

class LocalApp(object):
   def __init__(self, program):
     # code here
   def start(self):
     # code here
   def stop(self):
     # code here

class RemoteApp(object):
   def __init__(self, program, host):
     # code here
   def start(self):
     # code here
   def stop(self):
     # code here

def Application(program, host=None):
   if host is None:
     return LocalApp(program)
     return RemoteApp(program, host)

> To me forwarding each call in the Application class looks a little bit
> redundant 

Indeed !-)

> and I would like to get rid of it. 

cf above. But in case you need to do proper delegation in Python, the 
magic words are "__getattr__" and "__setattr__". Here's a very basic 
example of using __getattr__ - using __setattr__ is a bit more tricky, 
but you'll find all relevant documentation in the FineManual(tm):

class Wrapper(object):
   def __init__(self, obj):
     self.__obj = obj

   def __getattr__(self, name):
     return getattr(self.__obj, name)

> Does anyone have any
> comments or suggestions? Can metaclass programming come to rescue?

May I suggest that you first learn Python bases before going into 
complex things ?

And FWIW, googling for "KISS" might help too !-)


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