[Tutor] functions in Python

[Danny Yoo]

On Mon, 17 Apr 2006, Payal Rathod wrote:

> What is the difference between,
>
>>>> def f(x):
> ...     return x
> ...
>>>> f(4)
> 4
>
>>>> def f(x):
> ...     print x
> ...
>>>> f(4)
> 4
>
> Both give same results.

Clarification.  Both "show" the same results from the interpreter.  From 
what you see so far, there's no difference.


One way to see the difference is to write functions that use functions. 
A elementary-school example might be to find the hypotenuse of a right 
triangle.  Given two legs of lengths 'a' and 'b', we know that the 
hypotenuse has this relationship:

     hypotenuse = sqrt(square(a) + square(b))

Let's write this out as a set of functions:

###################################################################
def hypo(a, b):
     """Given lengths of lengths a and b, returns the hypotenuse."""
     return sqrt(square(a) + square(b))

def square(x):
     return x * x

def sqrt(x):
     return x**(0.5)
###################################################################

Experiment with this.  See what happens if you replace a return statement 
with a print statement in square() or sqrt().


The key idea here is that print only shows us its output: it doesn't do 
anything else with it.  It's what's called a "side-effect".  And 
sometimes, we don't care about seeing something as much as we do getting 
the related value back.  Above, hypo does not care what square looks like 
when it's printed to you, but it cares about getting the real value so it 
can do its computations.

Does this make sense?