[Tutor] arg exceptions for scrips

[kevin parks]

Below is my little copy thing which does nothing but copy a file. It works, 
but i would like be able to use it as a script and a module. So here i have it
wired up for the usual __name__, run as main thing. The problem is that the cp()
func takes 2 args. So i tried to patch it up so that when i typed:

% cp.py foo.txt bar.txt

it executed the cp() function and passes the 2 command line arguments to
the func itself. I don't know if this is the standard way to do this or my
kludge but it works, except the if len(sys.argv) < 2: part. What i want it to
is complain if it doesn't get exactly 2 arguments and i want to to say: 
"Usage: infile outfile" or something close to that and then exit. But it doesn't
Python itself complains if the args aren't right and i get:

[localhost:~/scripts] kevin% python cp.py scales.txt 
Traceback (most recent call last):
  File "cp.py", line 28, in ?
    cp(sys.argv[1],sys.argv[2])
IndexError: list index out of range


for less than 2 args (and nothing yet for more than 2). Is there no
way to tell python to chill and print the exception and usage i want?
This way if someone other than me executes this script they can know what
the expected arguements are rather than knowing:
cp(sys.argv[1],sys.argv[2])
IndexError: list index out of range

Which is useless for the end user.




#!/usr/bin/env python

import os
import sys

def cp(infilename, outfilename):
	"""this will copy a file exactly, args are: 'infile' and 'outfile'"""
	infile = open(infilename, 'r')
	f = open(outfilename, 'w')
	for aLine in infile.xreadlines() :
		f.write( aLine )
	infile.close()
	f.close()

# if used as a script we have to pass the args from the
# shell command-line to the function above. If there are
# not exactly 2 args, complain and quit.

if __name__ == '__main__':
	if len(sys.argv) < 2:
		print "Usage:", sys.argv[0], "infile outfile"
		sys.exit(1)
	else:
		cp(sys.argv[1],sys.argv[2])
	


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