Using object variable as a recursive method argument in a Python class

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?

aganders3 · Accepted Answer · 2012-01-03 02:20:02Z

6

Default parameters are evaluated when the method is defined. Thus, it's essentially "too late" to use a member value defined in __init__. What you should do is set the default value to None and test for this in the body of the function:

class nonsense(object):
    def __init__(self, val):
        self.val = val
    def factorial(self, n=None):
        if n is None:
            n = self.val
        elif n <= 1:
            return 1

        return n*self.factorial(n-1)

answered Jan 3, 2012 at 2:20

aganders3

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Comments

Ethan Furman · Accepted Answer · 2012-01-03 02:41:53Z

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...")

Óscar López · Accepted Answer · 2012-01-03 02:29:38Z

1

The reason for the strange behavior is that the def statement only gets executed once, when the function is defined. Thus the initializer value only gets created once, at a time when there isn't yet a self reference.

As an alternative, try this:

class nonsense(object):
  def __init__(self, val):
    self.val = val
  def factorial(self, n=None):
    return self.factorial_aux(n if n is not None else self.val)
  def factorial_aux(self, n):
    if n <= 1:
        return 1
    return n * self.factorial(n-1)

The above solution only tests once if the n parameter has the default value (None) and after that, it returns the result of calling factorial_aux (which does the actual work) with the appropriate argument.

edited Jan 3, 2012 at 2:29

answered Jan 3, 2012 at 2:23

Óscar López

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Comments

Arnab Ghosal · Accepted Answer · 2012-01-03 09:37:11Z

0

why not write is simply as this?

class fact1():  
    def fact2(self, num):
        if num==1:
            return 1
        ss=num*self.fact2(num-1)
        return ss

exx=fact1()
print exx.fact2(5)

Output 120

answered Jan 3, 2012 at 9:37

Arnab Ghosal

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1 Comment

Ethan Furman Over a year ago

Because this isn't simple and clear. Compare fact(5) vs fact1().fact2(5).

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