Generate .ics iCal event for TLS Certificate Expiration

#!/usr/bin/env python

import sys
from datetime import datetime,timedelta
import pytz
import uuid
try:
    from OpenSSL import crypto
except:
    print ("Do you have pyOpenSSL? Try 'pip install pyOpenSSL'.")
    exit(1)
try:
    from icalendar import Calendar,Event,Alarm
except:
    print ("Do you have Python icalendar installed? Try 'pip install icalendar'.")
    exit(1)

class InvalidCertificateException(Exception):
    pass


class CertificateExpiration(Calendar):
    PRODID='-//Certificate expiration tracker'
    VERSION="2.0"

    def __init__(self,certfile,local_tz_str,email_addr):
        super(self.__class__, self).__init__()
        try:
            cert=crypto.load_certificate(crypto.FILETYPE_PEM, file(certfile).read())
        except crypto.Error:
            print("Error loading certificate file: %s" % certfile)
            raise InvalidCertificateException()

        commonName=str(cert.get_subject().commonName)
        expires_datetime=pytz.utc.localize(datetime.strptime(cert.get_notAfter(),"%Y%m%d%H%M%SZ"))
        local_tz=pytz.timezone(local_tz_str)
        local_expires=expires_datetime.astimezone(local_tz)

        self.expires=local_expires.date()
        nextday=self.expires+timedelta(days=1)
        self.add('prodid',self.PRODID)
        self.add('version',self.VERSION)

        event=Event()
        event.add('summary', ("Certificate Expiration: %s" % commonName))
        event.add('dtstart',self.expires)
        event.add('dtend',nextday)
        event.add('uid',uuid.uuid4())
        event.add('dtstamp',datetime.now())


        #Human readable time string with time zone (reflecting daylight savings time)
        #to add to calendar event description

        local_expires_str=local_expires.strftime("%Y-%m-%d, %-I:%M %p %Z")
        description="TLS Certificate CN: %s\r\n" % commonName
        description+="expires on: %s\r\n" % local_expires_str
        event.add('Description',description)

        #Email alarms are considered unsafe and are removed by Calendar.app
        # emailAlarm=Alarm()
        # emailAlarm.add('trigger',timedelta(days=-30))
        # emailAlarm.add('action','EMAIL')
        # emailAlarm.add('attendee',email_addr)
        # emailAlarm.add('summary',"Certificate expires in 30 days: %s" % commonName)
        # emailAlarm.add('description',"This is a reminder that the TLS certificate for %s expires on: %s" % (commonName,local_expires_str))
        # emailAlarm.add('uid',uuid.uuid4())
        # event.add_component(emailAlarm)

        displayAlarm=Alarm()
        #this ends up being 7 days, since the "all day" event starts at T-1day
        displayAlarm.add('trigger',timedelta(days=-6))
        displayAlarm.add('description','Certificate expires in 7 days: %s' % commonName)
        displayAlarm.add('action','DISPLAY')
        displayAlarm.add('uid',uuid.uuid4())
        event.add_component(displayAlarm)
        self.add_component(event)
        print self

    def display(self):
        return self.to_ical() #.replace('\r\n','\n').strip()

    def write(self,filename):
        open(filename,"wb").write(self.to_ical()    )


def main(cert_filename,local_tz_str,ics_filename,email_addr):
    try:
        cert_expiration=CertificateExpiration(cert_filename,local_tz_str,email_addr)
    except InvalidCertificateException:
        exit(1)
    cert_expiration.write(ics_filename)

if __name__ == '__main__':
    tz_str="America/Los_Angeles"
    certfile=sys.argv[1]
    email_addr=sys.argv[2]
    main(certfile,tz_str,"./Cert Expiration.ics",email_addr)