[Tutor] Question about Functions

[Dino Bektešević]

> Date: Tue, 10 Sep 2013 13:34:50 -0400
> From: novo shot <novoshot at gmail.com>
> To: tutor at python.org
> Subject: [Tutor] Question about Functions
> Message-ID:
>         <CAGk5SVGPVL-1xgEukXCcRUmmYJG_Aq+mh+wnR=ot5AJBCHt8bA at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Dear tutors:
>
> The following is an example I found in the Raspberry Pi for Dummies book:
>
> #function test
>
> def theFunction(message):
>     print "I don't get ", message
>     return "ARRRGH!"
>
> theFunction("this")
>
> result=theFunction("this either")
> print "reply is: ", result
>
> ---------------------------------------------
> The result of this code looks like this:
>
> I don't get  this
> I don't get  this either
> reply is:  ARRRGH!
>
> ----------------------------------------------
> Here's what I don't understand:
>
> When I declare a variable to be equal as the fucntion
> (result=theFunction("this either")) is Python also executing the function?
>
> The way I see it, I only called for the function once before printing
> ARRRGH!! Then after that I declared a variable and then I print.
>
> This is how I expected the result to look like:
>
> I don't get this
> reply is: I don't get this either
> ARRRGH!
>
> -----------------------------------------
>
> Can you help me understand? I can't move forward until I understand how
> Python solves this code.
>
> Thanks in advance

Function in Python work very much the same way like they do in most
programming languages. When working with them you can't really "feel"
the difference.
There is a big difference, however, "underneath" (just always keep in
mind everything in Python is an object, the book should mention that
repeatedly).
Since you didn't mention I have to assume you didn't do any
programming before python.

In the case of big C's (C, C++, C#) I was always thought that I should
look at a function as a variable. In particular the exact same
variable you return.
I.e. if I have a function:
      >>> def add1(int_number):
                      return number+1
you can look at function add1 as if it's an integer because it returns
an integer.
The same applies to your example, which you can see if you do some
introspection:
      >>> type(result)
      <type 'str'>
So you see your result is nothing more then a string! Basically
whatever you return in your function will be assigned to the variable
you return it to. Because you return "AARGH" in your function and
assign the return value to result:
     >>> result=theFunction("thisEither")
the variable result will become "AARGH". So this line basically amounts to:
     >>> result= "AARGH"

This is a special case because this function always returns the same
thing. That's not usually the case with functions.
I.e. let's return to my add1 function. Output (that part behind the
return statement) of add1 function will change depending on the input
number:
      >>> add1(3)
      4
      >>> added = add1(5)
      >>> print added
      6
What you can also see from the above example is that when I explicitly
assigned a variable to a function [added = add1(5)] the result of the
function did not print out, but the function must have executed
because variable added has a value of '6'.
So function executes every time you call on it, but prints out value
of "return" only when you don't specify a "container" that will hold
it's output.

I'm pretty sure you're confused here because you have a print
statement in your function. Print statement calls on your standard
output that works "over" anything and pretty much "whenever", it's
kind of special that way. If I changed my example function add1 to:
      >>> def add1(int_number):
                      print "Working, hold your horses...."
                      return number+1
then my output from before would be:
      >>> add1(3)
      Working, hold your horses....
      4
      >>> added = add1(5)
     Working, hold your horses....
      >>> print added
      6
This is the main difference between return and print, think of return
like it defaults to print if there is no value to which it can assign
what it returned. That is also why you always see line "I don't get
(this/this either)" printed every time you call on your function.
      >>> theFunction("this")
      I don't get this
      ARRGHHH
      >>> result=theFunction("this either")
      I don't get this either
      >>> print "reply is: ", result
      reply is AARGGGHHHH
I think it should be pretty clear by now how it works.....

On another note apparently ugly with an f in front is a bad word
around here, my apologies I'm fairly new around these places and was
inquiring for your help not even a week ago and don't really know how
things work. But I am willing to help out like you did me, does that
count? Also I'm not a programmer so I imagine analogies when I program
and may be off point sometimes, hope I didn't make too many people
cringe because of that...

Regards,
Dino