None shown in output

[Dan Sommers]

On Fri, 22 Jun 2012 11:42:28 +0800
Xander Solis <xrsolis at gmail.com> wrote:

> Noob here with a newbie question. I'm reading and working on the
> exercise of the book, Learn Python the Hard way 2.0. When I use this
> code, I get "None" on the output. My question is why does this happen?
> 
> def get_numbers(first_num, second_num, operator):
> 
>     if operator == 'add':
>         print first_num + second_num
>     elif operator == 'minus':
>         print first_num - second_num
>     elif operator == 'divide':
>         print first_num / second_num
>     elif operator == 'multiply':
>         print first_num * second_num

This function prints a result, but doesn't return anything.

> print "%r" % (get_numbers(1, 2, 'minus'))
> print "%r" % (get_numbers(1+3, 2+9, 'add'))
> print "%r" % (get_numbers(10, 2, 'divide'))

This:

    (get_numbers(1, 2, 'minus'))

calls get_numbers and evaluates to None (because get_numbers doesn't
return anything).

Then this:

    "%r" % (get_numbers(1, 2, 'minus'))

formats that None into the string "None."

Finally, this:

    print "%r" % (get_numbers(1, 2, 'minus'))

prints that string.

> Output:
> 
> C:\code\python>ex19.py
> -1

This "-1" comes from get_numbers.

> None

This "None" comes from the print statement that called get_numbers.

Contrast your code with this snippet:

    def add(x, y):
        return x + y

    print "%r" % (add(3, 2))

HTH,
Dan

-- 
Μὴ μοῦ τοὺς κύκλους τάραττε -- Αρχιμηδησ
Do not disturb my circles. -- Archimedes

Dan Sommers, http://www.tombstonezero.net/dan