Question

I am trying to write a recursive function within a class, but have some trouble using an object var as a method argument:

class nonsense(object):
  def __init__(self, val):
    self.val = val
  def factorial(self, n=self.val):
    if n<=1: return 1
    return n*self.factorial(n=n-1)

The code above generates the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in nonsense
NameError: name 'self' is not defined

But if I don’t refer to self.val, the error disappears, although having to specify n is redundant:

class nonsense(object):
  def __init__(self, val):
    self.val = val
  def factorial(self, n):
    if n<=1: return 1
    return n*self.factorial(n=n-1)

What is the right way of doing this?

Answer 1

As you have discovered, you cannot have self.xxx in the header — rather have None and then correct in the body:

def factorial(self, n=None):
    if n is None: n = self.val
    if n<=1: return 1
    return n*self.factorial(n=n-1)

The reason is that when the class object is being created there is no self; besides globals(), the only names defined when Python gets to factorial are __module__, and __init__.

As an experiment to prove this to yourself, try this:

class TestClassCreation(object):
    print("Started creating class")
    print("names so far: %s" % vars())

    def __init__(self):
        pass
    print("now we have %s" % vars())

    def noop(self, default=None):
        print("this gets run when noop is called")
    print("and now have %s" % vars())
    print()

    print("and now we'll fail...")
    def failure(self, some_arg=self.noop):
        pass
    print("we never get here...")