Add element polygon shortcut as a property

Closes #161 (closed)

arkindex_worker/models.py

@@ -5,7 +5,7 @@ Wrappers around API results to provide more convenient attribute access and IIIF
 
 import tempfile
 from contextlib import contextmanager
-from typing import Generator, Optional
+from typing import Generator, List, Optional
 
 from PIL import Image
 from requests import HTTPError
@@ -93,6 +93,19 @@ class Element(MagicDict):
             url += "/"
         return "{}full/{}/0/default.jpg".format(url, size)
 
+    @property
+    def polygon(self) -> List[float]:
+        """
+        Access an Element's polygon.
+        This is a shortcut to an Element's polygon, normally accessed via
+        its zone field via `zone.polygon`. This is mostly done
+        to facilitate access to this important field by matching
+        the [CachedElement][arkindex_worker.cache.CachedElement].polygon field.
+        """
+        if not self.get("zone"):
+            raise ValueError("Element {} has no zone".format(self.id))
+        return self.zone.polygon
+
     @property
     def requires_tiles(self) -> bool:
         """