Python Returning None?

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Python Returning None?

Postby 1337 » Tue Sep 18, 2012 6:49 am UTC

Having problems with a python script. Could anyone tell me why this is returning a None statement when I call this function?

Code: Select all

def power():
    x = int(raw_input('Enter a number: '))
    y = int(raw_input('Enter the power: '))
    c = 0
    if x < 0:
        print 'The number you entered', x, 'is a negative number'
    elif x == 0:
        print 'You can\'t use', x, 'as the number'
    else:
        c = x**y
        return c

print power()


Output:

Code: Select all

>>>
Enter a number: 0
Enter the power: 5
You can't use 0 as the number
None


Any help would be appreciated...

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Xanthir
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Re: Python Returning None?

Postby Xanthir » Tue Sep 18, 2012 8:07 am UTC

Because it automatically returns None if you don't tell it to return something else. Your code only explicitly returns a value in the "good" case - in the others, it just prints something, and then exits the if stack and sees the end of the function, which causes it to infer a "return None;" at the end.
(defun fibs (n &optional (a 1) (b 1)) (take n (unfold '+ a b)))

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Mat
Posts: 414
Joined: Fri Apr 21, 2006 8:19 pm UTC
Location: London

Re: Python Returning None?

Postby Mat » Tue Sep 18, 2012 8:08 am UTC

Yup. Printing is not the same as returning a value from a function. It is just a side effect that calling the function has.

If x is 0 or below, there are no return statements, so the return value is None.

A more pythonic way would be to handle your bad input using exceptions, like this

Code: Select all

def power():
  ...
  if x <= 0:
    raise ValueError()

try:
  print power()
except ValueError:
  print 'you're doing it wrong'


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