Python 类继承 AttributeError - 为什么？怎么修？

Question

关于 SO 的类似问题包括:this one和 this .我也通读了我能找到的所有在线文档，但我仍然很困惑。如果你能提供帮助，我将不胜感激。

我想在我的 CastSpell 类 lumus 方法中使用 Wand 类的 .wandtype 属性。但我不断收到错误消息“AttributeError:‘CastSpell’对象没有属性‘wandtype’。”

此代码有效:

class Wand(object):
    def __init__(self, wandtype, length):
        self.length = length 
        self.wandtype = wandtype

    def fulldesc(self):
        print "This is a %s wand and it is a %s long" % (self.wandtype, self.length) 

class CastSpell(object):
    def __init__(self, spell, thing):
        self.spell = spell 
        self.thing = thing

    def lumus(self):
        print "You cast the spell %s with your wand at %s" %(self.spell, self.thing) 

    def wingardium_leviosa(self): 
        print "You cast the levitation spell."

my_wand = Wand('Phoenix-feather', '12 inches') 
cast_spell = CastSpell('lumus', 'door') 
my_wand.fulldesc()  
cast_spell.lumus() 

此代码尝试继承，但没有。

class Wand(object):
    def __init__(self, wandtype, length):
        self.length = length 
        self.wandtype = wandtype

    def fulldesc(self):
        print "This is a %s wand and it is a %s long" % (self.wandtype, self.length) 

class CastSpell(Wand):
    def __init__(self, spell, thing):
        self.spell = spell 
        self.thing = thing

    def lumus(self):
        print "You cast the spell %s with your %s wand at %s" %(self.spell, self.wandtype, self.thing)   #This line causes the AttributeError! 
        print "The room lights up."

    def wingardium_leviosa(self): 
        print "You cast the levitation spell."

my_wand = Wand('Phoenix-feather', '12 inches') 
cast_spell = CastSpell('lumus', 'door') 
my_wand.fulldesc()  
cast_spell.lumus() 

我试过使用 super() 方法无济于事。非常感谢您帮助理解 a) 为什么类继承在这种情况下不起作用，b) 如何让它起作用。

Answer 1

简单来说，你在继承自它的类中覆盖了 Wand.__init__，所以 CastSpell.wandtype 永远不会在 CastSpell 中设置>。除此之外，my_wand 无法将信息传递给 cast_spell，因此您对继承的作用感到困惑。

无论您如何操作，您都必须以某种方式将length 和wandtype 传递给CastSpell。一种方法是将它们直接包含到 CastSpell.__init__ 中:

class CastSpell(Wand):
    def __init__(self, spell, thing, length, wandtype):
        self.spell = spell 
        self.thing = thing
        self.length = length
        self.wandtype = wandtype

另一种更通用的方法是将这两个传递给基类自己的 __init__():

class CastSpell(Wand):
    def __init__(self, spell, thing, length, wandtype):
        self.spell = spell 
        self.thing = thing
        super(CastSpell, self).__init__(length, wandtype)

另一种方法是停止让 CastSpell 从 Wand 继承(CastSpell 是一种 Wand 吗？或者 Wand 做的事情？)，而是让每个 Wand 都能够在其中包含一些 CastSpell:而不是“is-a”(a CastSpell 是一种 Wand)，尝试“has-a”(Wand 有 Spell)。

这里有一个简单但不是很好的方法来使用魔杖存储法术:

class Wand(object):
    def __init__(self, wandtype, length):
        self.length = length
        self.wandtype = wandtype
        self.spells = {} # Our container for spells. 
        # You can add directly too: my_wand.spells['accio'] = Spell("aguamenti", "fire")

    def fulldesc(self):
        print "This is a %s wand and it is a %s long" % (self.wandtype, self.length)

    def addspell(self, spell):
        self.spells[spell.name] = spell

    def cast(self, spellname):
        """Check if requested spell exists, then call its "cast" method if it does."""
        if spellname in self.spells: # Check existence by name
            spell = self.spells[spellname] # Retrieve spell that was added before, name it "spell"
            spell.cast(self.wandtype) # Call that spell's cast method, passing wandtype as argument
        else:
            print "This wand doesn't have the %s spell." % spellname
            print "Available spells:"
            print "\n".join(sorted(self.spells.keys()))


class Spell(object):
    def __init__(self, name, target):
        self.name = name
        self.target = target

    def cast(self, wandtype=""):
        print "You cast the spell %s with your %s wand at %s." % (
               self.name, wandtype, self.target)
        if self.name == "lumus":
            print "The room lights up."
        elif self.name == "wingardium leviosa":
            print "You cast the levitation spell.",
            print "The %s starts to float!" % self.target

    def __repr__(self):
        return self.name

my_wand = Wand('Phoenix-feather', '12 inches')
lumus = Spell('lumus', 'door')
wingardium = Spell("wingardium leviosa", "enemy")

my_wand.fulldesc()
lumus.cast() # Not from a Wand! I.e., we're calling Spell.cast directly
print "\n\n"

my_wand.addspell(lumus) # Same as my_wand.spells["lumus"] = lumus
my_wand.addspell(wingardium)
print "\n\n"

my_wand.cast("lumus") # Same as my_wand.spells["lumus"].cast(my_wand.wandtype)
print "\n\n"
my_wand.cast("wingardium leviosa")
print "\n\n"
my_wand.cast("avada kadavra") # The check in Wand.cast fails, print spell list instead
print "\n\n"