Using getopt parse and validate command line arguments

Question

I am learning getopt in Python to parse and validate command line inputs:

#!/usr/bin/python
'''\nMailing Script
Usage: python script.py -y <year> -c <type_user>"
Arguments:
 -y, --year any value from 2008 to 2013
 -c, --campaign any value in:
 '80c', '80d', 'allowances', 'late_filing', 
 'exempt_income', 'other_sources' 
Flags:
 -h help
''' 
import sys, getopt
first_yr, last_yr = (2008, 2013)
campaign_type = (
 '80c', '80d', 'allowances', 'late_filing', 
 'exempt_income', 'other_sources',
)
def usage():
 print sys.exit(__doc__)
def parse_arguments(argv):
 try:
 opts, args = getopt.getopt(argv[1:], "y:c:", ["year=", "campaign="])
 except getopt.GetoptError:
 usage() 
 for opt, arg in opts:
 if opt == '-h':
 usage()
 elif opt in ("-y", "--year"):
 target_year = arg
 # customized exception
 try:
 if not target_year in map(str, range(first_yr, last_yr+1)):
 raise Exception()
 except:
 sys.exit("Argument Error: Invalid year passed. \n" 
 "One valid year is within (%s, %s)" % (first_yr, last_yr)
 ) 
 elif opt in ("-c", "--campaign"):
 campaign = arg.lower()
 try: 
 if not campaign in campaign_type:
 raise Exception()
 except Exception, e:
 sys.exit("Argument Error: Invalid 'Campaign type' passed. \n" 
 "A valid value can be any of %s" % str(campaign_type)
 )
 # to avoid 'UnboundLocalError' Exception when one argument not passed
 try:
 target_year, campaign
 except UnboundLocalError, u: 
 usage() 
 print 'Year : ', target_year 
 print 'campaign: ', campaign 
 # return argument values
 return (target_year, campaign)
if __name__ == "__main__":
 target_year, campaign = parse_arguments(sys.argv)

_{Presently I didn't learn 16.4. argparse. I will learn it in the next step.}

I want to improve my script as follows:

1. All arguments are mandatory. Presently if the user misses some argument then the script raises an UnboundLocalError exception. I then catch it and call usage(). Is it a correct way? Should I use len(argv) in some ways to handle this?

2. I check for correct values of arguments. If any argument value is wrong then I explicitly raise an exception and print an error message in the except clause. I am again not sure whether it's a preferable way to write this kind of code. I feel it adds one more level of nested blocks.

Some runs of this script:

$ python opt.py -y 2012 -c 80c
Year : 2012
campaign: 80c

Correct as I want:

$ python opt.py -year=2012 
Argument Error: Invalid year passed. 
One valid year is within (2008, 2013)

I think I should print message something like "insufficient arguments passed" or "use --year=2012 or -y 2012".

Answer 1

I think you have too many try/excepts. More than one of them raise and catch their own exceptions. I've made some changes that improves readability.

For clarity the new code created a new exception class and raises it when appropriate.

Also using getopt's own handler for bogus input. The else in the main loop is redundant but it can also serve as a fallback in case you make a mistake by allowing an arg but forget to process it.

Pay attention to the wording: if target_year not in map( ...

reads better than: if not target_year in map( ...

#!/usr/bin/python
'''\nMailing Script
Usage: python script.py -y <year> -c <type_user>"
Arguments:
 -y, --year any value from 2008 to 2013
 -c, --campaign any value in:
 '80c', '80d', 'allowances', 'late_filing', 
 'exempt_income', 'other_sources' 
Flags:
 -h help
''' 
import sys
import getopt
first_yr, last_yr = (2008, 2013)
campaign_type = (
 '80c', '80d', 'allowances', 'late_filing',
 'exempt_income', 'other_sources',
)
class ArgumentError(Exception):
 pass
def usage():
 print sys.exit(__doc__)
def parse_arguments(argv):
 target_year = None
 campaign = None
 try:
 opts, args = getopt.getopt(argv[1:], "y:c:", ["year=", "campaign="])
 except getopt.GetoptError as err:
 # print help information and exit:
 print str(err) # will print something like "option -a not recognized"
 usage()
 for opt, arg in opts:
 if opt == '-h':
 usage()
 elif opt in ("-y", "--year"):
 target_year = arg
 if target_year not in map(str, range(first_yr, last_yr + 1)):
 raise ArgumentError("Argument Error: Invalid year passed. \n "
 "One valid year is within (%s, %s)" % (first_yr, last_yr))
 elif opt in ("-c", "--campaign"):
 campaign = arg.lower()
 if campaign not in campaign_type:
 raise ArgumentError("Argument Error: Invalid 'Campaign type' passed. "
 "A valid value can be any of %s" % str(campaign_type))
 else:
 raise ArgumentError("Bad argument: I don't know what %s is" % arg)
 if target_year is None or campaign is None:
 raise ArgumentError("You need to supply both -y and -c")
 print 'Year : ', target_year 
 print 'campaign: ', campaign 
 # return argument values
 return target_year, campaign
if __name__ == "__main__":
 target_year, campaign = parse_arguments(sys.argv)