Using getopt parse and validate command line arguments

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

I have written my following code to parse command line arguments using getopt:

#!/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 next step}

I want to improve my script as follows:

1. All arguments are mandatory. Presently if user miss some argument then script raise UnboundLocalError exception I caught it and call usage(). Is it correct way? Should I use len(argv) someways to handle this?

2. I check for correct values of arguments, if any argument value is wrong then I explicitly rise an Exception and print error message in except clause. I am again not sure whether its preferable way to write this kind of code. I feel it adds one more level of nested block.

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".

Please suggest me improvements in my script.

If there are any other tips on how to improve this code please let me know.