Callable as the default argument to dict.get without it being called if the key exists

Question

I am trying to provide a function as the default argument for the dictionary\'s get function, like this

def run():
   print \"RUNNING\"

test = {\'store\':1         


        

Answer 1

If you only know what the callable is likely to be at he get call site you could subclass dict something like this

    class MyDict(dict):

        def get_callable(self,key,func,*args,**kwargs):
            '''Like ordinary get but uses a callable to 
            generate the default value'''

            if key not in self:
                val = func(*args,**kwargs)
            else:
                val = self[key]
            return val

This can then be used like so:-

     >>> d = MyDict()
     >>> d.get_callable(1,complex,2,3)
     (2+3j)
     >>> d[1] = 2
     >>> d.get_callable(1,complex,2,3)
     2
     >>> def run(): print "run"
     >>> repr(d.get_callable(1,run))
     '2'
     >>> repr(d.get_callable(2,run))
     run
     'None'

This is probably most useful when the callable is expensive to compute.