[Tutor] design advice for function

[Brian van den Broek]

Christopher Spears said unto the world upon 2005-12-18 01:30:
 > I got my function to work!  It takes arguments and
 > adds them:

Hi Christopher,

great!


 > def adder(**args):
 >     argsList = args.values()
 >     sum = argsList[0]
 >     for x in argsList[1:]:
 >         sum = sum + x
 >     return sum
 >
 > print adder()
 > print "---"
 > print adder(a=5)
 > print "---"
 > print adder(a=5,b=6)
 > print "---"
 > print adder(a=5,b=6,c=7)
 > print "---"
 > print adder(ugly=7, good=6, bad=5)
 > print "---"
 >
 > However, if I run the above code.  I get an error:
 >
 > Traceback (most recent call last):
 >   File "C:\Documents and Settings\Christopher
 > Spears\My
 > Documents\programming\PythonScripts\Part4\Ex04\adder.py",
 > line 8, in -toplevel-
 >     print adder()
 >   File "C:\Documents and Settings\Christopher
 > Spears\My
 > Documents\programming\PythonScripts\Part4\Ex04\adder.py",
 > line 3, in adder
 >     sum = argsList[0]
 > IndexError: list index out of range
 >
 > This is caused by the line: print adder().  Obviously
 > if adder() doesn't receive any arguments, it can't
 > build the lists resulting in an IndexError.

Right. You are also going to have a like problem with the next chunk 
of you function:

for x in argsList[1:]:
          sum = sum + x

(If you fix the adder() problem alone, you will still have a 
adder(one_arg) problem.)


 > What is
 > the best way to solve this?  Should I write some
 > syntax into the function to check for arguments?
 > Should I just write a seperate function to check for arguments?

Well, what you like adder() to do?

It is not uncommon for operations of a sequence to have a separate 
clause for the special case of the empty sequence. So, you could do:

if not argsList:
     return what_Christopher_wants

If you do that, can you think of a way to get around the 
adder(one_arg) problem? Hint: do you really need to treat the first 
element of argsList as a special case?

Have a think, and if you get stuck, post again.

Best,

Brian vdB