The very basics of the plugin API

src/tixlplugins/timerestriction/signals.py

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+from django.dispatch import receiver
+from django.utils.timezone import now
+
+from tixlbase.signals import determine_availability
+
+from .models import TimeRestriction
+
+
+@receiver(determine_availability)
+def availability_handler(sender, **kwargs):
+    # Handle the signal's input arguments
+    item = kwargs['item']
+    variations = kwargs['variations']
+    cache = kwargs['cache']  # NOQA
+    context = kwargs['context']  # NOQA
+
+    # Fetch all restriction objects applied to this item
+    restrictions = list(TimeRestriction.objects.filter(
+        items__in=(item,),
+    ).prefetch_related('variations'))
+
+    # If we do not know anything about this item, we are done here.
+    if len(restrictions) == 0:
+        return variations
+
+    # IMPORTANT:
+    # We need to make a two-level deep copy of the variations list before we
+    # modify it, becuase we need to to copy the dictionaries. Otherwise, we'll
+    # interfere with other plugins.
+    variations = [d.copy() for d in variations]
+
+    # Walk through all variations we are asked for
+    for v in variations:
+        # If this point is reached, there ARE time restrictions for this item
+        # Therefore, it is only available inside one of the timeframes, but not
+        # without any timeframe
+        available = False
+        price = None
+        # Walk through all restriction objects applied to this item
+        for restriction in restrictions:
+            applied_to = list(restriction.variations.all())
+
+            # Only take this restriction into consideration if it either
+            # is directly applied to this variation OR is applied to all
+            # variations (e.g. the applied_to list is empty)
+            if len(applied_to) > 0:
+                if 'variation' not in v or v['variation'] not in applied_to:
+                    continue
+
+            if restriction.timeframe_from <= now() and restriction.timeframe_to >= now():
+                # Selling this item is currently possible
+                available = True
+                # If multiple time frames are currently active, make sure to
+                # get the cheapest price:
+                if restriction.price is not None and (price is None or restriction.price < price):
+                    price = restriction.price
+
+        v['available'] = available
+        v['price'] = price
+
+    return variations