[1.5.x] Fixed #19366 -- Prevented GEOSIndexError when comparing geometries

Thanks Craig de Stigter for the report and collaboration on the
patch.
Backport of cc3c4a9d5 from master.

django/contrib/gis/geos/mutable_list.py

@@ -149,27 +149,30 @@ class ListMixin(object):
         return self
 
     def __eq__(self, other):
-        for i in range(len(self)):
+        olen = len(other)
+        for i in range(olen):
             try:
                 c = self[i] == other[i]
-            except IndexError:
+            except self._IndexError:
-                # must be other is shorter
+                # self must be shorter
                 return False
             if not c:
                 return False
-        return True
+        return len(self) == olen
 
     def __lt__(self, other):
-        slen = len(self)
+        olen = len(other)
-        for i in range(slen):
+        for i in range(olen):
             try:
                 c = self[i] < other[i]
-            except IndexError:
+            except self._IndexError:
-                # must be other is shorter
+                # self must be shorter
-                return False
+                return True
             if c:
                 return c
-        return slen < len(other)
+            elif other[i] < self[i]:
+                return False
+        return len(self) < olen
 
     ### Public list interface Methods ###
     ## Non-mutating ##

django/contrib/gis/geos/tests/test_geos.py

@@ -451,6 +451,21 @@ class GEOSTest(unittest.TestCase, TestDataMixin):
             self.assertEqual(poly.wkt, Polygon(*tuple(r for r in poly)).wkt)
             self.assertEqual(poly.wkt, Polygon(*tuple(LinearRing(r.tuple) for r in poly)).wkt)
 
+    def test_polygon_comparison(self):
+        p1 = Polygon(((0, 0), (0, 1), (1, 1), (1, 0), (0, 0)))
+        p2 = Polygon(((0, 0), (0, 1), (1, 0), (0, 0)))
+        self.assertTrue(p1 > p2)
+        self.assertFalse(p1 < p2)
+        self.assertFalse(p2 > p1)
+        self.assertTrue(p2 < p1)
+
+        p3 = Polygon(((0, 0), (0, 1), (1, 1), (2, 0), (0, 0)))
+        p4 = Polygon(((0, 0), (0, 1), (2, 2), (1, 0), (0, 0)))
+        self.assertFalse(p4 < p3)
+        self.assertTrue(p3 < p4)
+        self.assertTrue(p4 > p3)
+        self.assertFalse(p3 > p4)
+
     def test_multipolygons(self):
         "Testing MultiPolygon objects."
         prev = fromstr('POINT (0 0)')

django/contrib/gis/geos/tests/test_mutable_list.py

@@ -363,6 +363,7 @@ class ListMixinTest(unittest.TestCase):
 
         pl, ul = self.lists_of_len()
         self.assertEqual(pl, ul, 'cmp for equal')
+        self.assertFalse(ul == pl + [2], 'cmp for not equal')
         self.assertTrue(pl >= ul, 'cmp for gte self')
         self.assertTrue(pl <= ul, 'cmp for lte self')
         self.assertTrue(ul >= pl, 'cmp for self gte')
@@ -377,6 +378,14 @@ class ListMixinTest(unittest.TestCase):
         self.assertTrue(ul < pl + [2], 'cmp')
         self.assertTrue(ul <= pl + [2], 'cmp')
 
+        # Also works with a custom IndexError
+        ul_longer = ul + [2]
+        ul_longer._IndexError = TypeError
+        ul._IndexError = TypeError
+        self.assertFalse(ul_longer == pl)
+        self.assertFalse(ul == ul_longer)
+        self.assertTrue(ul_longer > ul)
+
         pl[1] = 20
         self.assertTrue(pl > ul, 'cmp for gt self')
         self.assertTrue(ul < pl, 'cmp for self lt')

docs/ref/contrib/gis/geos.txt

@@ -656,6 +656,17 @@ is returned instead.
 
    Returns the number of interior rings in this geometry.
 
+.. admonition:: Comparing Polygons
+
+    Note that it is possible to compare ``Polygon`` objects directly with ``<``
+    or ``>``, but as the comparison is made through Polygon's
+    :class:`LineString`, it does not mean much (but is consistent and quick).
+    You can always force the comparison with the :attr:`~GEOSGeometry.area`
+    property::
+
+        >>> if poly_1.area > poly_2.area:
+        >>>     pass
+
 Geometry Collections
 ====================