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how to let argument be optional falling back to certain integer

I just wrote

def f(y, N, k = None):
  k = k or (N - 1)
  return k

I was surprised to find out that 0 == False, so f(7, 31, 0) produces 31.

I'd like 0 to be a valid choice for k.  

How do you guys let k be an optional argument such that it defaults to 
N - 1?  

Thank you.