Alex Hall wrote:
> Hi all,
> I am now trying to allow my classes, all of which subclass a single
> class (if that is the term), to provide optional arguments. Here is
> some of my code:
>
> class Craft():
> def __init__(self,
> name,
> isAircraft=False,
> id=helpers.id(),
> hits=0,
> weapons=[]):
> self.name=name
> self.id=id
> self.hits=hits
> self.weapons=weapons
> self.isAircraft=isAircraft
> #end def
> #end class
>
>
> #now a class for each type of craft in the game
> #each will subclass the Craft class, but each also has its own settings
>
> class Battleship(Craft):
> def __init__(self,
> name,
> maxHits):
> Craft.__init__(self, name)
> self.maxHits=maxHits
> self.length=maxHits #in Battleship, each ship's length is the same
> as how many times it can be hit
> #end def
> #end class
>
>
> I want to be able to say something like
> b=Battleship("battleship1", 4, weapons=["missile1","missile2"])
> When I do that, though, I get a traceback on the above line saying
> "type error: __init__() got an unexpected keyword argument 'weapons'".
> What did I do wrong / what do I need to change so that any Craft
> (Battleship, Carrier, and so on) can optionally provide a list of
> weapons, or any other arguments to which I assign default values in my
> Craft class? I hope this makes sense.
You have to repeat the arguments for the base class in the Battleship
initializer. Battleship(...) will only invoke Battleship.__init__(...), not
Craft.__init__(...) -- so you have to do that explicitly inside
Battleship.__init__() while passing along all parameters it is supposed to
process. As simplified example:
class Craft(object):
def __init__(self, name, id=None, weapons=None):
if id is None:
id = helpers.id()
if weapons is None:
weapons = []
self.name = name
self.id = id
self.weapons = weapons
class Battleship(Craft):
def __init__(self, name, id=None, weapons=None, max_hits=None):
Craft.__init__(self, name, id, weapons)
self.max_hits = max_hits
Notice how using None to signal that a value was not provided simplifies
things; without it you'd have to put a meanigful default for weapons into
both the Craft and Battleship class. If you want a unique id for every Craft
providing helpers.id() as a default is just wrong; default arguments are
calculated once and you would end up with the same id for every Craft
instance.
Peter
--
http://mail.python.org/mailman/listinfo/python-list
