Python: Inheritance and default values in init

Question

I am trying to make a class with generic __init__ values, but have defaults for its subclasses, as so:

class Enemy:

 def __init__(self, difficulty, power, MaxHP, magic, MaxMP, speed, name):
    self.power = power + 2*difficulty
    self.HP = self.MaxHP = MaxHP + 5*difficulty
    self.magic = magic + 2* difficulty
    self.MP = self.MaxMP = MaxMP + 5*difficulty
class Goblin(Enemy):
 def __init_(self, difficulty = 1, power = 1, MaxHP = 5, magic = 1, MaxMP = 5, speed = 5, name = "Goblin"):
    super(Goblin, self).__init__(self, power, MaxHP, magic, MaxMP, speed, name)

However, when I try to make a Goblin object without the full number of default values (like, I'll just put in a value for difficulty), it tells me I need the full 8 arguments even though the rest are given default values. Is there any reason I can't do that or am I doing something wrong here?

You're missing an underscore from Goblin's __init__. So you should copy and paste the code that is causing the problem. And the full traceback, too. Python's tracebacks are usually very helpful. — Gary Kerr
– Gary Kerr, Commented Nov 10, 2011 at 1:02

wim · Accepted Answer · 2011-11-10 01:39:53Z

5

Because you called super(Goblin, self).__init__(self, power, MaxHP, magic, MaxMP, speed, name) without difficulty. You probably also want to inherit like class Enemy(object) to make sure Enemy is a new-style class if you're on 2.x (which I guess you must be, considering the old way that you've used super).

Here's a simpler example:

class Animal(object):
  def __init__(self, talk):
    print '__init__ Animal: ', talk

class Cat(Animal):
  def __init__(self, talk='meow'):
    print '__init__ Cat'
    super(Cat, self).__init__(talk)

if __name__ == '__main__':
  tom = Cat()

Outputs :

__init__ Cat
__init__ Animal:  meow

Edit:

Well if the following doesn't work, perhaps you have old class definitions cached in your interpreter (try running it on a fresh interpreter).

class Enemy(object):
  def __init__(self, difficulty, power, MaxHP, magic, MaxMP, speed, name):
    self.power = power + 2*difficulty
    self.HP = self.MaxHP = MaxHP + 5*difficulty
    self.magic = magic + 2* difficulty
    self.MP = self.MaxMP = MaxMP + 5*difficulty
    print 'Raaarghh!! I am the formidable {}.'.format(name)

class Goblin(Enemy):
  def __init__(self, difficulty=1, power=1, MaxHP=5, magic=1, MaxMP=5, speed=5, name="Goblin"):
    super(Goblin, self).__init__(difficulty, power, MaxHP, magic, MaxMP, speed, name)

if __name__ == '__main__':
  g = Goblin(name='user1038783 goblin')

edited Nov 10, 2011 at 1:39

answered Nov 10, 2011 at 0:49

wim

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4 Comments

limasxgoesto0 Over a year ago

Hah, the things you miss when you think you tried everything. And thank you for the Object part.

limasxgoesto0 Over a year ago

I tried running the program again (as opposed to my test driver), and even with difficulty there I still get the same error "TypeError: init takes exactly 8 arguments, 2 given"

wim Over a year ago

OK, check the double underscore in the __init__ , and don't pass in self explicitly because it gets done implicitly.

limasxgoesto0 Over a year ago

The __init__() was spelled correctly, and I tried all combos of removing the self from (Goblin, self) and (self, difficulty, power...), and I still have the same error.

Ben · Accepted Answer · 2011-11-10 02:22:19Z

This code works for me:

class Enemy(object):
    def __init__(self, difficulty, power, MaxHP, magic, MaxMP, speed, name):
        self.power = power + 2*difficulty
        self.HP = self.MaxHP = MaxHP + 5*difficulty
        self.magic = magic + 2* difficulty
        self.MP = self.MaxMP = MaxMP + 5*difficulty

class Goblin(Enemy):
    def __init__(self, difficulty = 1, power = 1, MaxHP = 5, magic = 1, MaxMP = 5, speed = 5, name = "Goblin"):
        super(Goblin, self).__init__(difficulty, power, MaxHP, magic, MaxMP, speed, name)

The ways I had to change yours to get it to work:

Fix misspelled def __init__ in Goblin.
- Symptom: Goblin() raised TypeError: __init__() takes exactly 8 arguments (1 given), as Goblin had not defined an __init__ method, so it had inherited the one without defaults from Enemy
Change Enemy into a new-style class by inheriting from object
- Symptom: I got TypeError: must be type, not classobj on the call to super(); I'm not sure whether older versions of Python would have allowed it or triggered a different error, but I know old-style classes have different MRO (method resolution order) rules than new-style classes, and I believe this could make super screwy anyway.
Remove the second self from the call to super(Goblin, self).__init__(self, ...)
- Symptom: self is automatically passed to super(Class, self).some_method(...), so putting it in there yourself is like calling Enemy.__init__(self, self, difficulty, power, ...).
Added difficulty to the call to super(Goblin, self).__init__(...)
- Symptom: You were getting difficulty defaulted in Goblin.__init__, but then not passing the value up to Enemy.__init__.

I think that was about it.

Ah yes, wim mentioned the init being mistyped but I missed that again when I checked it over. Thanks to both of you, it's working now.

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