Fixed #27557 -- Casted GEOSGeometry only when necessary

Thanks Pete Flugstad for the report, and Tim Graham for the review.

django/contrib/gis/geos/geometry.py

@@ -95,13 +95,15 @@ class GEOSGeometry(GEOSBase, ListMixin):
             self.srid = srid
 
         # Setting the class type (e.g., Point, Polygon, etc.)
+        if type(self) == GEOSGeometry:
             if GEOSGeometry._GEOS_CLASSES is None:
                 # Lazy-loaded variable to avoid import conflicts with GEOSGeometry.
                 from .linestring import LineString, LinearRing
                 from .point import Point
                 from .polygon import Polygon
                 from .collections import (
-                GeometryCollection, MultiPoint, MultiLineString, MultiPolygon)
+                    GeometryCollection, MultiPoint, MultiLineString, MultiPolygon,
+                )
                 GEOSGeometry._GEOS_CLASSES = {
                     0: Point,
                     1: LineString,

tests/gis_tests/geos_tests/test_geos.py

@@ -1308,6 +1308,25 @@ class GEOSTest(SimpleTestCase, TestDataMixin):
         self.assertEqual(args, (Point(0, 0), MultiPoint(Point(0, 0), Point(1, 1)), poly))
         self.assertEqual(kwargs, {})
 
+    def test_subclassing(self):
+        """
+        GEOSGeometry subclass may itself be subclassed without being forced-cast
+        to the parent class during `__init__`.
+        """
+        class ExtendedPolygon(Polygon):
+            def __init__(self, *args, **kwargs):
+                data = kwargs.pop('data', 0)
+                super(ExtendedPolygon, self).__init__(*args, **kwargs)
+                self._data = data
+
+            def __str__(self):
+                return "EXT_POLYGON - data: %d - %s" % (self._data, self.wkt)
+
+        ext_poly = ExtendedPolygon(((0, 0), (0, 1), (1, 1), (0, 0)), data=3)
+        self.assertEqual(type(ext_poly), ExtendedPolygon)
+        # ExtendedPolygon.__str__ should be called (instead of Polygon.__str__).
+        self.assertEqual(str(ext_poly), "EXT_POLYGON - data: 3 - POLYGON ((0 0, 0 1, 1 1, 0 0))")
+
     def test_geos_version(self):
         """Testing the GEOS version regular expression."""
         from django.contrib.gis.geos.libgeos import version_regex