input_file = 'in_file.csv'
output_file = 'out_file.csv'


def main():

    try:
        with open(input_file, 'rb') as in_csv, open(output_file , 'wb') as out_csv:
            writer = csv.writer(out_csv, delimiter='t')
            reader = csv.reader(in_csv, delimiter='t')
            for row in reader:
                # many lines of code...
    except IOError as e:
        print "Error: cannot open file"
        if e.errno == errno.EACCES:
            print "tPermission denied."
            print "tError message: {0}".format(e)
            sys.exit()
        # Not a permission error.
        print "tDoes file exist?"
        print "tError message: {0}".format(e)
        sys.exit()


if __name__ == '__main__':
    main()

def process(csv_reader, csv_writer):
    for row in csv_reader:
        # many lines of code...

def main(input_filename, output_filename):
    try:
        with open(input_filename, 'rb') as in_csv, open(output_filename , 'wb') as out_csv:
            writer = csv.writer(out_csv, delimiter='t')
            reader = csv.reader(in_csv, delimiter='t')
            process(reader, writer)
    except IOError as e:
        print >> sys.stderr, "Error: cannot open file"
        if e.errno == errno.EACCES:
            print >> sys.stderr, "tPermission denied."
            print >> sys.stderr, "tError message: {0}".format(e)
            sys.exit(1)
        # Not a permission error.
        print >> sys.stderr, "tError message: {0}".format(e)
        sys.exit(1)
    except Exception as other_exception:
        print >> sys.stderr, "Error: " + str(other_exception)
        sys.exit(2)

if __name__ == '__main__':
    main('in_file.csv', 'out_file.csv')