[Tutor] 'return' statements

[Daniel Yoo]

On Sun, 13 Aug 2000, Albert Antiquera wrote:

> I've finished the calculator program I was making thanks to the help
> of Daniel Yoo. One statement that continue to elude me to fully
> understand it is the 'return' statement. On some text they said
> 'return' is a "go back to the start of the function" another said that
> it is a "terminator" of a function. Can somebody explain it and please
> provide some examples like when to use ' 0 ' and ' 1 ' after a return
> statement and also this usage (Thanks, Daniel for this very useful
> function!!):


No problem.  About 'return': All functions will give back a value to the
"calling" function.  Even when you don't say anything like "return",
functions will at least send back the "None" object.  Take a look:

###
>>> def doNothing():
...   pass
... 
>>> x = doNothing()
>>> print x
None
###


What a 'return' statement allows you to do is specify what sort of value
you want your functions to give back.  For example:

###
>>> def square(x): return x * x
... 
>>> square(5)
25
>>> square(square(5))
625
###


Also, 'return' does a quick exit out of the function.

###
>>> def testReturn():
...   return "I've returned!"
...   print "I'll never get here."
... 
>>> x = testReturn()
>>> x
"I've returned!"
###



What I was doing in:


>   def readNum():
>      x = raw_input("Enter a value for X:")
>      y = raw_input("Enter a value for Y:")
>      return (float (x),float(y))   <<<<<<<<<<<<what exactly is return doing here?????


was returning a list of values (technically a tuple).  You might not have
been introducted to lists yet; wait till you get there, and you'll
understand what's happening.



> another example:
> 
> def julian_leap(y =2000):
>     if (y%4) == 0:
>         return 1 <<<< I guess this says "true" but I'm not really sure
>     return 0 <<< what is this returning????


It's literally either returning the number 1 or 0.  There isn't a distinct
"true" or "false" data type; Python conveniently reuses 1 and 0 to
represent Truth.

###
>>> 'A' == 'A'
1
>>> 'A' == 'B'
0
###


So you can abuse the language by doing something like:

###
>>> ('A' == 'A') + ('A' == 'B')
1
###