Django - how to count the number of hours, minutes, seconds (TimeField)?

Question

Django

All good!
There is a model:

class Blablabla(models.Model):
    start_t = models.TimeField(u"С", blank=True, null=True)
    end_t  = models.TimeField(u"По", blank=True, null=True)

You need to count the number of hours, minutes, seconds.
Maybe a templatetag?

Answer 1

If you need a universal solution for comparing any two fields containing time, then template tag is probably quite an adequate solution. If you only need to compare two specific fields of this model, it will be much easier for you to add the corresponding methods to the model and access them from the template ({{ blablabla_obj.hours }}, {{ blablabla_obj.minutes }}, {{ blablabla_obj.seconds }}).
However, your question is ill-posed. Do you really need to know the difference between these two values? In the form HH:MM:SS?
The author pointed out that minutes and seconds are not specified at all, and end_t is always greater than start_t. Thus, it is enough to compare start_t.hour and end_t.hour.
You can do this by adding the appropriate method to the model:

class Blablabla(models.Model):
    start_t = models.TimeField(u"From", blank=True, null=True)
    end_t  = models.TimeField(u"To", blank=True, null=True)

    def period_duration(self):
        """
        Returns the length of the period between
        start_t and end_t, in hours.

        This is precise as long as both values
        have 0 as minute/second.
        """
        return self.end_t.hour - self.start_t.hour

Further, respectively, if blablabla_obj is an object of the Blablabla model, then you can get the difference between start_t and end_t using blablabla_obj.period_duration() in the code or {{ blablabla_obj.period_duration }} in the template.
Test:

class BlablablaTestCase(TestCase):
    def setUp(self):
        start_t = datetime.time(14)
        end_t = datetime.time(18)
        self.blablabla_obj = Blablabla.objects.create(
            start_t=start_t,
            end_t=end_t
        )

    def test_period_calculation(self):
        """
        Tests that the period_duration method of the Blablabla
        model correctly compares start_t and end_t.
        """
        self.assertEqual(self.blablabla_obj.period_duration(), 4)

    def tearDown(self):
        self.blablabla_obj.delete()

If you need to count hours, minutes, and seconds, then the period_duration method can be rewritten like this:

def period_duration(self):
    """
    Returns the length of the period between
    start_t and end_t, in the following format:

    HH:MM:SS
    """
    total_seconds = (
        (
            self.end_t.hour*3600
            +
            self.end_t.minute*60
            +
            self.end_t.second
        ) - (
            self.start_t.hour*3600
            +
            self.start_t.minute*60
            +
            self.start_t.second
        )
    )
    hour, total_seconds = divmod(total_seconds, 3600)
    minute, total_seconds = divmod(total_seconds, 60)
    second = total_seconds
    return "%02d:%02d:%02d" % (hour, minute, second)