[Tutor] arg exceptions for scrips

[George Gates]

>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])

The problem is the test of the length should be less than 3, not less than 
2.  If you executed with only one argument the length of sys.argv would be 
2, sys.argv[0] for the script name, sys.argv[1] for the argument:

ggates@ggates ggates]$ more arg_count.py

#!/usr/bin/env python
import sys
print "The number of arguments are %s: %s" % (len(sys.argv),sys.argv)

[ggates@ggates ggates]$ ./arg_count.py file1

The number of arguments are 2: ['./arg_count.py', 'file1']

[ggates@ggates ggates]$



At 11:32 PM 7/13/2001 +0900, kevin parks wrote:
>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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>_______________________________________________
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George H. Gates
System Administrator - OSOS Implementation

Phone: (315) 453-2912 x5671
Email: ggates@appliedtheory.com
Web: http://ggates.appliedtheory.com