Python Metaclasses with Class Inheritance

Question

So I've recently come to an understanding with inheritance for classes and have come to appreciate their uses. So I began coding a concrete example up for myself.

I realized as I was making multiple classes for different weapons that it would be easier if I could have a class that generated these classes for me. I'm a little stuck on how to get this done.

I have search around and it gets confusing and theoretical quick. This example on this site (http://en.wikibooks.org/wiki/Python_Programming/Metaclasses) under "Class Factories" seems to be what I need, but I seem to be missing something.

Here's what I have:

class item(object):
    def __init__(self, item_name, item_type):
        self.item_type = item_type
        self.item_name = item_name

    def get_item_name(self):
        return self.item_name

    def get_item_type(self):
       return self.item_type

    def __str0__(self):
        return "%s is a %s" % (self.item_name, self.item_type)

class weapon(item):
    def __init__(self, name, weapon_type, dmg_mod, atk_typ):
        item.__init__(self, name, "weapon")
        self.name = name
        self.weapon_type = weapon_type
        self.dmg_mod = dmg_mod
        self.atk_typ = atk_typ

    def get_wpn_type(self):
        return self.weapon_type

    def get_dmg_mod(self):
        return self.dmg_mod

    def get_atk_typ(self):
        return atk_typ

    def __str1__(self):
        return "%s is a %s with %s type damage at a %i modifier" % (self.name,
                                                                    self.weapon_type,
                                                                    self.atk_typ,
                                                                    self.dmg_mod)

def weapon_fact(weptype, dmg_type):
    class weptype(weapon):
        def __init__(self, name, dmg_mod):
            weapon.__init__(self, name, weptype, dmg_mod, dam_type)
            self.name = name

When I do

longbow = weapon_fact("Long Bow", "Pierce")
huntersbow = longbow("Hunter's Bow", 5)

I get this traceback

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    huntersbow = longbow("Hunter's Bow", 5)
TypeError: 'NoneType' object is not callable

And when I try to call longbow it has None assigned to it.

score 0 · Accepted Answer · answered Sep 20 '14 at 22:25

Your weapon_fact doesn't return anything, so the default None is returned instead.

You also want to avoid masking the weptype name:

def weapon_fact(weptype, dmg_type):
    class SpecificWeapon(weapon):
        def __init__(self, name, dmg_mod):
            weapon.__init__(self, name, weptype, dmg_mod, dam_type)
            self.name = name
    return SpecificWeapon

Python Metaclasses with Class Inheritance

1 Answers1