[Tutor] python - files

[Cameron Simpson]

Mats has mentioned the modules getopt and argparse etc. These are 
primarily aimed at option parsing ("-v", "-o foo"). Your situation 
occurs _after_ the option parsing (in your case, there are no options).

Alan has talked about explicitly checking the length of sys.argv, much 
as you are doing, or accessing the (missing) argument and catching an 
exception.

There's a middle ground, which is a little more flexible, which is to 
_consume_ the command line arguments. The argv array is a list, and can 
be modified. So:

  def main(argv):
    cmd = argv.pop(0)   # collect the command word
    badopts = False
    # mandatory first argument
    if not argv:
      print("%s: missing first argument" % cmd, file=sys.stderr)
      badopts = True
    else:
      first = argv.pop(0)
      # optional second argument
      if argv:
        second = argv.pop(0)    # explicit argument 2, use it
      else:
        second = None           # or some otherdefault
      if argv:
        print("%s: extra arguments: %r" % (cmd, argv), file=sys.stderr)
        badopts = true
    if badopts:
      print("%s: invalid invocation, aborting" % cmd, file=sys.stderr)
      return 2
    ... work with first and second ...

You can see here that we process the arguments from left to right, 
consuming the valid ones as we find them, and setting a flag for 
incorrect arguments. At the end we test the flag and abort if it is set.  
otherwise we process knowing we have valid values.

One important aspect of the above code is that you do not wire in an 
explicit length for sys.argv such as 2 or 3. That way you can easily 
change your code later if you want more arguments without running around 
adjusting such fixed numbers.

The other important aspect is usability: the above code complains about 
each issue it encounters, and finally quits with an additional message.  
In a real programme that addition message would include a "usage" 
message which describes the expected arguments.

Cheers,
Cameron Simpson <cs at cskk.id.au>