At the Forge - Dynamically Generated Calendars

The real solution, and one that makes life more interesting, is to create the iCalendar file dynamically when the user requests it. That is, our CGI program does not return the contents of an existing iCalendar file; instead, it creates an iCalendar file programmatically, returning it to the user's calendar client program.

At first glance, this might seem to be a simple task. After all, the iCalendar file format appears to be straightforward, so maybe we can code something together ourselves. But upon closer examination, we discover that creating an iCalendar file is easier said than done, particularly if we want to include recurring events.

Given the increasing popularity of the iCalendar standard and the plethora of open-source projects, I was surprised to discover the relative lack of attention that iCalendar has received from the biggest open-source programming communities. Part of my surprise was because iCalendar has been around for several years, is used by many companies and is supported by many calendar programs, from Novell's Evolution to Lotus Notes to Microsoft Outlook. This combination usually is a recipe for several different options, in several different programming languages.

I first looked at Perl, whose CPAN archive is renowned for its many modules, including many for Internet standards of various sorts. Although several Perl modules are available that parse iCalendar files, no up-to-date module exists for building them. Net::ICal::Libical was going to be a wrapper around the C-language libical library but was last released in a pre-alpha version, several years ago. Net::ICal was part of a project called ReefKnot, which also appears to have been abandoned.

Luckily, the Danish developer Max M (see the on-line Resources) recently decided to fill this gap and wrote a Python package that makes it easy to create an iCalendar file. I downloaded and installed the package on my computer without any trouble, and I found that it is quite straightforward to create a calendar with this package. Combined with our simple CGI program from before, we should be able to create and publish a calendar without any trouble.

I downloaded and installed the iCalendar package from the maxm.dk site. Unlike many modern Python packages, it doesn't install automatically. You must copy it manually to your system's site-packages directory, which on my Fedora Core 3 system is located at /usr/lib/python-2.3/site-packages.

As you can see in Listing 2, I was able to use this newly installed iCalendar package to create new objects of type Calendar and Event. The first thing I had to do was import the appropriate packages into the current namespace:

from iCalendar import Calendar, Event

#!/usr/bin/python

# Grab the CGI module
import cgi
from iCalendar import Calendar, Event
from datetime import datetime
from iCalendar import UTC # timezone

# Log any problems that we might have
import cgitb
cgitb.enable(display=0, logdir="/tmp")

# Send a content-type header to the user's browser
print "Content-type: text/calendar\n\n"

# Create a calendar object
cal = Calendar()

# What product created the calendar?
cal.add('prodid',
        '-//Python iCalendar 0.9.3//mxm.dk//')

# Version 2.0 corresponds to RFC 2445
cal.add('version', '2.0')

# Create one event
event = Event()
event.add('summary', 'ATF deadline')
event.add('dtstart',
          datetime(2005,3,11,8,0,0,tzinfo=UTC()))
event.add('dtend',
          datetime(2005,3,11,10,0,0,tzinfo=UTC()))
event.add('dtstamp',
          datetime(2005,3,11,0,10,0,tzinfo=UTC()))
event['uid'] = 'ATF20050311A@lerner.co.il'

# Give this very high priority!
event.add('priority', 5)

# Add the event to the calendar
cal.add_component(event)

# Ask the calendar to render itself as an iCalendar
# file, and return that file in an HTTP response!
print cal.as_string()

The Calendar and Event modules inside of the iCalendar package correspond to the entire iCalendar file and one event in that file, respectively. We thus create a single instance of the Calendar object and one Event object for each event that we might want to create.

We then can create the calendar object:

cal = Calendar()
cal.add('prodid',
        '-//Python iCalendar 0.9.3//mxm.dk//')
cal.add('version', '2.0')

The second and third lines here, in which we invoke cal.add(), allow us to add identifying data to our iCalendar file. The first of these allows us to tell the client software which program generated the iCalendar file. This is useful for debugging; if we consistently get corrupt iCalendar files from a particular software package, we can contact the author or publisher and report a bug. The second line, in which we add a version identifier, indicates which version of the iCalendar specification we are following. RFC 2445 indicates that we should give this field a value of 2.0 if we are going to follow that specification.

Now that we have created a calendar, let's create an event and give it a summary line to be displayed in the calendar program of anyone subscribing to this iCalendar file:

event = Event()
event.add('summary', 'ATF deadline')

Every event, as we have already seen in the file we examined, has three date/time fields associated with it: the starting date and time, dtstart; the ending date and time, dtend; and an indication of when this entry was added to the calendar, dtstamp. The iCalendar standard uses a strange if useful format for its dates and times, but the Event object knows how to work with those if we give it a datetime object from the standard datetime Python package. So, we can say:

event.add('dtstart',
          datetime(2005,3,11,14,0,0,tzinfo=UTC()))
event.add('dtend',
          datetime(2005,3,11,16,0,0,tzinfo=UTC()))
event.add('dtstamp',
          datetime(2005,3,11,0,10,0,tzinfo=UTC()))

Notice that the above three lines used UTC as the time zone. When the iCalendar file is displayed inside of a client Calendar application, it is shown with the user's local time zone, as opposed to UTC.

Once we have created the event, we need to give it a unique ID. When I say unique, I mean that the ID should be truly unique, across all calendars and computers in the world. This sounds trickier than it actually is. You can use a number of different strategies, including using a combination of the creation timestamp, IP address of the computer on which the event was created and a large random number. I decided to create a simple UID, but if you are creating an application to be shared across multiple computers, you probably should think about what sort of UIDs you want to create and then standardize on them:

event['uid'] = 'ATF20050311A@lerner.co.il'

Finally, we must give our event a priority, in the range of 0 through 9. An event with priority 5 is considered to be normal or average; urgent items get higher numbers and less-urgent items get lower ones:

event.add('priority', 5)

Once we have created our event, we attach it to the calendar object, which has been waiting for us to do something with it:

cal.add_component(event)

If we are so interested, we then could to add more events to the calendar. So long as each has a unique UID field, there won't be any problems.

Finally, we turn our Calendar object into an iCalendar file, using the as_string() method:

print cal.as_string()

Because print writes to standard output by default, and because CGI programs send their standard output back to the HTTP client, this has the effect of sending an iCalendar file back to whoever made the HTTP request. And because we have defined the MIME type to be of type text/calendar, the HTTP client knows to interpret this as a calendar and display it appropriately. If we look at the output ourselves, we see that it is indeed in iCalendar format:

BEGIN:VCALENDAR
PRODID:-//Python iCalendar 0.9.3//mxm.dk//
VERSION:2.0
BEGIN:VEVENT
DTEND:20050311T160000Z
DTSTAMP:20050311T001000Z
DTSTART:20050311T140000Z
PRIORITY:5
SUMMARY:ATF deadline
UID:ATF20050311A@lerner.co.il
END:VEVENT
END:VCALENDAR

Now, I must admit that this example is almost as contrived as the previous one. True, we have exploited the fact that we can generate a calendar dynamically, but this event was hard-coded into the program, making it impossible for a nonprogrammer to add, modify or delete the event. That said, we have taken an additional step toward the programmatic calculation of events and dates. The next step is to store the dates in a file or even in a relational database and to use our program to convert the information on the fly.