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
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