Python: Dynamically assign class methods

Question

Essentially this is what I want to accomplish:

class Move(object):
 def __init__(self, Attr):
 if Attr:
 self.attr = Attr
 if hasattr(self, "attr"):
 __call__ = self.hasTheAttr
 else:
 __call__ = self.hasNoAttr
 def hasNoAttr(self):
 #no args!
 def hasTheAttr(func, arg1, arg2):
 #do things with the args
 __call__ = hasNoAttr

I know that that doesn't work, it just uses hasNoAttr all the time. My first thought was to use a decorator, but I'm not all that familiar with them and I couldn't figure out how to base it from whether or not a class attribute existed or not.

Actual question part: How would I be able to deterministically make a function either x function or y function depending on a condition.

• Jon Clements: Could I ask what the use case is then?
• DanielCardin: this is for movement code, where the parent object may or may not have a collision attribute. In _call_, when you have the attribute, two arguments are needed, none are needed if its there's no attribute. I wanted to do it this way rather than having default arguments so that it would error if I forgot to give those arguments to the one's that needed it (whereas you wouldn't get such an error if I was using default arguments

Answer 1

You can't really do this sort of thing with __call__ -- with other (non-magic) methods, you can just monkey-patch them, but with __call__ and other magic methods you need to delegate to the appropriate method within the magic method itself:

class Move(object):
 def __init__(self, Attr):
 if Attr:
 self.attr = Attr
 if hasattr(self, "attr"):
 self._func = self.hasTheAttr
 else:
 self._func = self.hasNoAttr
 def hasNoAttr(self):
 #no args!
 def hasTheAttr(func, arg1, arg2):
 #do things with the args
 def __call__(self,*args):
 return self._func(*args)

• Jon Clements: Well - "you need to" is not exactly accurate - as you can "meta" this kind of thing, but this is the most sensible given lack of any further understanding into the intention of why?
• mgilson: @JonClements -- I'm not super comfortable with metaclasses yet -- Feel free to post a solution. I'm sure that I could learn a thing or two from it :)