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Use of factory pattern in Python?

Nick Craig-Wood nick at craig-wood.com
Thu Dec 7 05:30:04 EST 2006 
Gabriel Genellina <gagsl-py at yahoo.com.ar> wrote:
>  The basic idea is the same, but instead of a long series of 
>  if...elif...else you can use a central registry (a dictionary will 
>  do) and dispatch on the name. Classes act as their own factories.
> 
>  registry = {}
> 
>  class Base(object):
>       kind = "Unknown"
>  register(Base)
> 
>  class Gene(Base):
>       kind = "gene"
>       def __init__(self, *args, **kw): pass
>  register(Gene)
> 
>  class Intron(Base):
>       kind = "intron"
>       def __init__(self, *args, **kw): pass
>  register(Intron)
> 
>  def register(cls):
>       registry[cls.kind] = cls
> 
>  def factory(kind, *args, **kw):
>       return registry[kind](*args, **kw)
> 
>  If some arguments are always present, you can name them, you're not 
>  limited to use the generic *args and **kw.

If you don't want to use a register() function on each class you could
introspect like this (untested) to make the registry :-

registry = {}
for obj in sys.modules[__name__].__dict__.values():
    try:
        if issubclass(obj, Base):
            registry[kind] = obj
    except TypeError:
        pass

There might be a neater way of writing the above!

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick
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