[Django] Validate that each day of the week is used in up to one set of opening hours. Includes user–friendly error messages.

opening_hours_validation.py

from django.core.exceptions import ValidationError

WEEKDAYS = (
    (1, 'Monday'),
    (2, 'Tuesday'),
    (3, 'Wednesday'),
    (4, 'Thursday'),
    (5, 'Friday'),
    (6, 'Saturday'),
    (7, 'Sunday'),
)

def clean(self):
	"""Validate that this set of opening hours doesn't conflict with another."""
	# we'll never have the first two sets of opening hours on the first iteration
	last_hours = False
	for hours in self.location.hours:
		# skip the first iteration so we always have two sets of opening hours to analyse
		if last_hours == False:
			last_hours = hours
			continue
		# make ranges of the weekdays involved
		last_days = range(last_hours.weekday_from, last_hours.weekday_to)
		days = range(hours.weekday_from, hours.weekday_to)
		# for all days in last_days, collect it, if it's also in days
		overlap = [day for day in last_days if day in days]
		# a day cannot be in two sets of opening hours
		if overlap.count() > 0:
			# convert weekday numbers into a comma-separated list of weekdays
			if overlap.count() == 1:
				weekdays = WEEKDAYS[overlap[0]][1]
			elif overlap.count() == 2:
				weekday1 = WEEKDAYS[overlap[0]][1]
				weekday2 = WEEKDAYS[overlap[1]][1]
				weekdays = '%s and %s' % (weekday1, weekday2)
			else:
				last_day = overlap.pop()
				weekdays = ', '.join(WEEKDAYS[day][1] for day in overlap)
				weekdays = '%s, and %s' % (weekdays, last_day)
			message = 'Opening hours for %s are already set.' % weekdays
			raise ValidationError(message)
		# compare with this set of opening hours on the next iteration
		last_hours = hours