Django models __unicode__: How to return a value containing localized datetimes?

Question

I have a localized Django App, localisation works well and the configuration is ok…

For forms needs, I use __unicode__ method for model to render ModelChoiceFields, but how do I format localized date in the unicode return?

In this method I have no access to current timezone, how to display my TimeSpanList correctly to my users? Currently, it displays UTC. I tryed django.template.defaultfilters.date and Simon Charette's django.utils.formats.localize which didn't helped as they probably lacked contextual data…

class TimeSpan(models.Model):
    start = models.DateTimeField(_("start"))
    end = models.DateTimeField(_("end"))

    def __unicode__(self):
        return u"from UTC:{0} to UTC:{1}".format(self.start, self.end)

class TimeSpanChooserForm(forms.Form):
    time_span = forms.ModelChoiceField(
        label=_("time span"), queryset=TimeSpan.objects.all())

    def __init__(self, *args, **kwargs):
        self.request = kwargs.pop("request", None)
        super(TimeSpanChooserForm, self).__init__(*args, **kwargs)

How to know current locale without current request object to localize those datetimes? (If there is a way)

note: to me __unicode__ seems like the only way to display entries in ModelChoiceField.

note 2: to me, Yuji 'Tomita' Tomita comment is the best current answer but it lacks a usable exemple…

Answer 1

Thanks for reminding me about this question. It's a bit involved for a complete answer.

I've created a demo method that shows this in action. Run test() to see output.

The remaining challenge is getting the client timezone. This will probably be done with some JavaScript that adds a parameter to your GET/POST or adds a custom header to be read in the view.

Since this is an unknown, I just stubbed it out with a method that returns a random time zone. You should update it to reflect your client timezone retrieval method.

Here's a self documenting example:

Models

from django.utils.formats import localize


class TimeSpan(models.Model):
    start = models.DateTimeField("start")
    end = models.DateTimeField("end")

    def __unicode__(self):
        return u"from UTC:{0} to UTC:{1}".format(self.start, self.end)

Form

from django import forms

class TimeSpanChooserForm(forms.Form):
    time_span = forms.ModelChoiceField(
        label=("time span"), queryset=TimeSpan.objects.all())

    def __init__(self, *args, **kwargs):
        # get request and TZ from view. This is a special form..
        self.request = kwargs.pop("request", None)
        self.tz = kwargs.pop('tz', None)
        super(TimeSpanChooserForm, self).__init__(*args, **kwargs)

        # modify the choices attribute to add custom labels.
        self.fields['time_span'].choices = [
            self.make_tz_aware_choice(self.request, timespan) for timespan in self.fields['time_span'].queryset
        ]

    def make_tz_aware_choice(self, request, timespan):
        """ Generate a TZ aware choice tuple.
        Must return (value, label). In the case of a ModelChoiceField, (instance id, label).
        """
        start = timespan.start.replace(tzinfo=self.tz)
        end = timespan.end.replace(tzinfo=self.tz)
        label = "From {tz} {start} to: {tz} {end}".format(
            start=start,
            end=end,
            tz=self.tz,
        )
        return (timespan.id, label)

View

import random
import pytz

from django import http
from django import template

def view(request):
    """ Render a form with overridden choices respecting TZ
    """
    def get_tz_from_request(request):
        """ Get the client timezone from request.
        How you do this is up to you. Likely from JS passed as a parmeter.
        """
        random_timezone = random.choice(pytz.all_timezones)
        return pytz.timezone(random_timezone)

    form = TimeSpanChooserForm(request.POST or None, request=request, tz=get_tz_from_request(request))
    ctx = template.Context({
        'form': form,
    })
    rendered_template = template.Template("{{ form }}").render(ctx)
    return http.HttpResponse(rendered_template)


def test():
    from django.test import RequestFactory

    rf = RequestFactory()
    r = rf.get('/')

    for i in range(10):
        print str(view(r))