Gabriel Genellina wrote:
> At Thursday 7/12/2006 05:28, Nathan Harmston wrote:
>> so I was thinking of having a factory class to return the individual
>> objects for each row......ie
>>
>> class Factory():
>> # if passed a gene return a gene object
>> # if passed an intron return an intron object
>> # if passed an exom return an exon object
>>
>> Is this a good way of doing this, or is there a better way to do this
>> in Python, this is probably the way I d do it in Java.
>
> 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)
>
And by using a metaclass on Base you can add the wizardry to have the
register-function called automatically whenever a class is derived from
Base. I also tend to use the class name instead of a separate "kind"
attribute.
registry = {}
class MetaBase(type):
def __init__(cls, name, bases, dict):
registry[name] = cls
class Base(object):
__metaclass__ = MetaBase
class Gene(Base):
def __init__(self, *args, **kw): pass
class Intron(Base):
def __init__(self, *args, **kw): pass
def factory(kind, *args, **kw):
return registry[kind](*args, **kw)
That way you don't have to think about calling the register-function
each time you derive a new class.
Regards,
Jan
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