Using the Python iCalendar Library

An application I’m working on uses the icalendar library ( to generate an ICS file of upcoming events that can be linked to another calendar program (I’ve tried Outlook and Google Calendar). One problem I was having was that I was unable to generate time zone aware times. Mine were all naive – also called ‘floating’ times – that would appear the same regardless of the calendar program’s timezone.

What I was trying to do was to create a new time zone in the ICS file, and then append the TZID parameter to each date/time value. It was not happening.

So, I dug into the source code for icalendar. This library is well documented with tests, and I soon found a solution – include a tzinfo value when supplying the date/time and icalendar will convert it to UTC and append a ‘Z’ to the end of the time to indicate this. icalendar supplies a class called LocalTimezone that can be used for this purpose. I tried it, and it worked! Here’s a simplified version of my code:

    from icalendar import Calendar, Event
    from datetime import datetime 
    from icalendar import LocalTimezone   
    cal = Calendar()
    cal.add('version', '2.0')
    cal.add('prodid', '-//test file//')
    cal.add('X-WR-CALNAME','Test Calendar ' )
    lt = LocalTimezone() # we append the local timezone to each time so that icalendar will convert
                         # to UTC in the output
    for ent in queryset:        
        event = Event()
        event.add('summary', ent.event_name)
        event.add('dtstart', datetime.combine(ent.event_date,ent.start_time).replace(tzinfo=lt))
        event.add('dtend', datetime.combine(ent.stop_date,ent.stop_time).replace(tzinfo=lt))
        event.add('dtstamp', ent.updated_on.replace(tzinfo=lt))
        event['uid'] =  # should probably use a better guid than just the PK
        event.add('priority', 5)
    return cal

And this is a sample event from the output:

DTEND: 20100714T184500Z
SUMMARY:Gateway Production

Note that my 1:45pm event in the US Eastern time zone (EDT) shows as 1745 in UTC.

Custom Widget – My First Experience

I hadn’t needed to write a custom widget before today, but I needed to have disabled options included in a <select> list. After a little research and a LOT of trial and error, this is what I came up with.

from django.forms.widgets import Select, force_unicode, flatatt, escape, mark_safe, chain, conditional_escape

class SelectDisabled(Select):
    def __init__(self, attrs=None, choices=(), disabled=[]):
        super(SelectDisabled, self).__init__(attrs, choices)
        self.disabled = list(disabled)

    def render(self, name, value, attrs=None, choices=()):
        if value is None: value = ''
        final_attrs = self.build_attrs(attrs, name=name)
        output = [u'<select%s>' % flatatt(final_attrs)]
        options = self.render_options(choices, [value], self.disabled)
        if options:
        return mark_safe(u'\n'.join(output))

    def render_options(self, choices, selected_choices, disabled_choices):
        def render_option(option_value, option_label):
            option_value = force_unicode(option_value)
            option_label = (option_value in disabled_choices) and (force_unicode(option_label) + ' - SOLD OUT') or force_unicode(option_label)
            selected_html = (option_value in selected_choices) and u' selected="selected"' or ''
            disabled_html = (option_value in disabled_choices) and u' disabled="disabled"' or ''
            return u'<option value="%s"%s%s>%s</option>' % (
                escape(option_value), selected_html, disabled_html,
        # Normalize to strings.
        selected_choices = set([force_unicode(v) for v in selected_choices])
        disabled_choices = set([force_unicode(v) for v in disabled_choices])
        output = []
        for option_value, option_label in chain(self.choices, choices):
            if isinstance(option_label, (list, tuple)):
                output.append(u'<optgroup label="%s">' % escape(force_unicode(option_value)))
                for option in option_label:
                output.append(render_option(option_value, option_label))
        return u'\n'.join(output)

And here’s how I used it in the form:

class B2CRegistrationStartForm(Form): 
    def __init__(self, disabled, *args, **kwargs):
        super(B2CRegistrationStartForm, self).__init__(*args, **kwargs)
        if disabled:
            self.fields['event'].widget.disabled = disabled
    event = ModelChoiceField(widget=SelectDisabled(),queryset=Event.b2c_web_reg.all(),empty_label=None)  

And here is the form creation in

    disabled = []
    for e in Event.b2c_sold_out.values_list('id'):
    form = B2CRegistrationStartForm( data=request.POST or None, disabled=disabled)

(The list of disabled IDs needs to be in a list, not a list of tuples)

It works fine, and I learned about custom widgets, but I don’t feel like I was very Object Oriented in my approach. The __init__() method uses super() well, but the other two basically replace the methods in the parent class. When I update to a newer version of Django (from the current 1.0.4), I’ll have to check this carefully and probably redo it.

Anyone have suggestions on how I could better implement this feature?

Simple Search in Django

I started programming (in BASIC, Pascal, and COBOL) before the Web was available, and it’s amazing to see how much the craft has changed with instant access to thousands of references. However, sometimes I throw up a search and get back the perfect solution, and I’m frustrated that I couldn’t figure out something so simple. It’s just too easy to search instead of think.

My example today is an easy search in Django. I have read about Haystack, Solr, and other search engines that require a daemon, special database structures, and other special things, but these are overkill for a search on a relatively small dataset.

My search returned this post from Toast Driven that outlined a way to append together a few Q objects to return a search result that uses multiple terms and multiple fields. Genius!

Thanks to Daniel at Toast Driven.