Python "maximum recursion depth exceeded in comparison" with variable arguments. Works fine with lists, however

Question

I have the following function:

def lst(*l):
  if l==():return None
  else: return cons(l[0],lst(l[1:]))

When I run it, I get "maximum recursion depth exceeded in comparison". Curiously, when I add a warper which converts the parameter tuple into a list, everything works just fine:

def lst(*l):
  return _lst(list(l))

def _lst(l):
  if l==[]:return None
  else: return (l[0],_lst(l[1:]))

>>> lst(1,2)
(1, (2, None))

What is the problem and how to deal with this strange behavior?

Replace if l==():return None with if not l: return None. Also, what does cons do? You might be able to replace this with reduce. — IceArdor
– IceArdor, Commented Apr 18, 2014 at 16:21
Thanks for your advice, @IceArdor. cons is a constructor like that: def __init__(self,car,cdr):self.car=car;self.cdr=cdr. I tried to replace the recursion with reduce, but something went wrong (seems like reduce works "wrong" direction) and I have absolutely no idea how to get it the right way. As long as i have a working code, thanks to Samy Arous, I wish not to make it broken. Thank you anyway. — valplo
– valplo, Commented Apr 18, 2014 at 16:47

Samy Arous · Accepted Answer · 2014-04-18 16:23:38Z

4

you are missing a * when passing the parameters again to the function

def lst(*l):
  if l==():return None
  else: return cons(l[0],lst(*l[1:]))

You are passing the empty tuple as a first positional parameter which means that in the next recursion l is actually equal to ((),) (a tuple containing one empty tuple) when it should be ()

answered Apr 18, 2014 at 16:23

Samy Arous

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