Re: Inheritance Question
- From: Gabriel Genellina <gagsl-py@xxxxxxxxxxxx>
- Date: Sat, 11 Nov 2006 04:46:58 -0300
At Saturday 11/11/2006 03:31, Frank Millman wrote:
Continuing your analogy of animals, assume a class A with a 'walk'
method and an 'eat' method.
Most animals walk the same way, but a few don't, so I create a subclass
AW and override the walk method.
Most animals eat the same way, but a few don't, so I create a subclass
AE and override the eat method.
How do I create an instance of an animal that both walks and eats
differently?
This is how I do it at present.
class A(object): # walks normally, eats normally
def walk(self):
normal walk
def eat(self):
normal eat
class AW(A): # walks differently, eats normally
def walk(self):
different walk
class E(object): # abstract class
def eat(self):
different eat
class AE(E,A): # walks normally, eats differently
pass
class AWE(E,AW): # walks differently, eats differently
pass
So I use multiple inheritance instead of subclassing to override the
eat method. It works, but it feels ugly. Is there a cleaner solution?
Answer 1) Move *both* ways of walk, and *both* ways of eating, to another base class. Best when this is pure behavior - no new attributes are involved.
class A(object): # does not know how to walk, neither how to eat
def walk(self):
raise NotImplemented
def eat(self):
raise NotImplemented
class WalkNormallyCapability(object): # knows how to walk normally
def walk(self):
normal walk
class WalkDifferentCapability(object): # knows how to walk different
def walk(self):
different walk
class EatNormallyCapability(object): # knows how to eat normally
def eat(self):
normal eat
class EatDifferentCapability(object): # knows how to eat different
def eat(self):
different eat
class AWnEn(A,WalkNormallyCapability,EatNormallyCapability): # walks normally, eats normally
pass
class AWdEn(A,WalkDifferentCapability,EatNormallyCapability): # walks different, eats normally
pass
....etc.
The xxxCapability classes are usually referred as "mixin class" - they add a new capability to an existing class, without adding new attributes (they could, but initialization gets more complicated, you must rewrite __init__ and so...). They may inherit from a common base class, too, but that's not required in Python.
Answer 2) Use an instance of another class to define how to walk and how to eat. Advantage: it can later be modified at runtime (strategy pattern).
class A(object):
def __init__(self, walker, eater):
self.walker=walker
self.eater=eater
def walk(self):
self.walker.walk()
def eat(self):
self.eater.eat()
class NormalWalker(object): # knows how to walk normally
def walk(self):
normal walk
class DifferentWalker(object): # knows how to walk different
def walk(self):
different walk
class NormalEater(object): # knows how to eat normally
def eat(self):
normal eat
class DifferentEater(object): # knows how to eat different
def eat(self):
different eat
a=A(NormalWalker(), DifferentEater())
or define a factory function when you decide which walker an which eater to use based on its arguments. As above, they may inherit from a common base class (Walker, Eater, by example).
--
Gabriel Genellina
Softlab SRL
__________________________________________________
Correo Yahoo!
Espacio para todos tus mensajes, antivirus y antispam ¡gratis! ¡Abrí tu cuenta ya! - http://correo.yahoo.com.ar
.
- Follow-Ups:
- Re: Inheritance Question
- From: Frank Millman
- Re: Inheritance Question
- References:
- Inheritance Question
- From: Jackson
- Re: Inheritance Question
- From: Frank Millman
- Inheritance Question
- Prev by Date: Re: UnboundLocalError
- Next by Date: Re: Py3K idea: why not drop the colon?
- Previous by thread: Re: Inheritance Question
- Next by thread: Re: Inheritance Question
- Index(es):
Relevant Pages
|
|
