diff --git a/python/BUILD b/python/BUILD
index 7c4088fe22..4cbc17add1 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -28,3 +28,19 @@ cc_binary(
         "@python_headers",
     ],
 )
+
+# Copy the extension into the location recognized by Python.
+genrule(
+    name = "message_ext",
+    srcs = [":message"],
+    outs = ["google/protobuf/pyext/_message.cpython-39-x86_64-linux-gnu.so"],
+    cmd = "cp $< $@",
+)
+
+py_test(
+    name = "minimal_test",
+    srcs = ["minimal_test.py"],
+    data = [":message_ext"],
+    imports = ["."],
+    legacy_create_init = False,
+)
diff --git a/python/descriptor.c b/python/descriptor.c
index b700fbbcc2..767c368494 100644
--- a/python/descriptor.c
+++ b/python/descriptor.c
@@ -56,11 +56,16 @@ static PyObject *PyUpb_DescriptorBase_New(PyTypeObject *subtype, PyObject *args,
 static PyObject *PyUpb_DescriptorBase_NewInternal(PyTypeObject *type,
                                                   const void *def,
                                                   PyObject *pool) {
-  PyUpb_DescriptorBase *base = PyObject_New(PyUpb_DescriptorBase, type);
-  base->pool = pool;
-  base->def = def;
-  Py_INCREF(pool);
-  PyUpb_ObjCache_Add(def, &base->ob_base);
+  PyUpb_DescriptorBase *base = (PyUpb_DescriptorBase*)PyUpb_ObjCache_Get(def);
+
+  if (!base) {
+    base = PyObject_New(PyUpb_DescriptorBase, type);
+    base->pool = pool;
+    base->def = def;
+    Py_INCREF(pool);
+    PyUpb_ObjCache_Add(def, &base->ob_base);
+  }
+
   return &base->ob_base;
 }
 
@@ -79,11 +84,6 @@ static void PyUpb_DescriptorBase_Dealloc(PyUpb_DescriptorBase *self) {
 // FieldDescriptor
 // -----------------------------------------------------------------------------
 
-typedef struct {
-  PyObject_HEAD
-  upb_fielddef *fielddef;
-} PyUpb_FieldDescriptor;
-
 static PyObject *PyUpb_FieldDescriptor_GetType(PyUpb_DescriptorBase *self,
                                                void *closure) {
   return PyLong_FromLong(upb_fielddef_descriptortype(self->def));
@@ -94,6 +94,11 @@ static PyObject *PyUpb_FieldDescriptor_GetLabel(PyUpb_DescriptorBase *self,
   return PyLong_FromLong(upb_fielddef_label(self->def));
 }
 
+static PyObject *PyUpb_FieldDescriptor_GetNumber(PyUpb_DescriptorBase *self,
+                                                  void *closure) {
+  return PyLong_FromLong(upb_fielddef_number(self->def));
+}
+
 static PyGetSetDef PyUpb_FieldDescriptor_Getters[] = {
   /*
   { "full_name", (getter)GetFullName, NULL, "Full name"},
@@ -107,8 +112,8 @@ static PyGetSetDef PyUpb_FieldDescriptor_Getters[] = {
   { "cpp_type", (getter)PyUpb_FieldDescriptor_GetCppType, NULL, "C++ Type"},
   */
   { "label", (getter)PyUpb_FieldDescriptor_GetLabel, NULL, "Label"},
+  { "number", (getter)PyUpb_FieldDescriptor_GetNumber, NULL, "Number"},
   /*
-  { "number", (getter)GetNumber, NULL, "Number"},
   { "index", (getter)GetIndex, NULL, "Index"},
   { "default_value", (getter)GetDefaultValue, NULL, "Default Value"},
   { "has_default_value", (getter)HasDefaultValue},
@@ -148,10 +153,10 @@ static PyType_Slot PyUpb_FieldDescriptor_Slots[] = {
 };
 
 static PyType_Spec PyUpb_FieldDescriptor_Spec = {
-  PYUPB_MODULE_NAME ".FieldDescriptor",      // tp_name
-  sizeof(PyUpb_FieldDescriptor),             // tp_basicsize
+  PYUPB_MODULE_NAME ".FieldDescriptor",
+  sizeof(PyUpb_DescriptorBase),
   0,                                    // tp_itemsize
-  Py_TPFLAGS_DEFAULT,                   // tp_flags
+  Py_TPFLAGS_DEFAULT,
   PyUpb_FieldDescriptor_Slots,
 };
 
diff --git a/python/minimal_test.py b/python/minimal_test.py
new file mode 100644
index 0000000000..b9303f4911
--- /dev/null
+++ b/python/minimal_test.py
@@ -0,0 +1,29 @@
+"""A bare-bones unit test, to be removed once upb can pass existing unit tests."""
+
+
+import unittest
+from google.protobuf.pyext import _message
+
+class TestMessageExtension(unittest.TestCase):
+
+    def test_descriptor_pool(self):
+        serialized_desc = b'\n\ntest.proto\"\x0e\n\x02M1*\x08\x08\x01\x10\x80\x80\x80\x80\x02:\x15\n\x08test_ext\x12\x03.M1\x18\x01 \x01(\x05'
+        pool = _message.DescriptorPool()
+        file_desc = pool.AddSerializedFile(serialized_desc)
+        self.assertEqual("test.proto", file_desc.name)
+        ext_desc = pool.FindExtensionByName("test_ext")
+        self.assertEqual(1, ext_desc.number)
+
+        # Test object cache: repeatedly retrieving the same descriptor
+        # should result in the same object
+        self.assertIs(ext_desc, pool.FindExtensionByName("test_ext"))
+
+
+    def test_lib_is_upb(self):
+        # Ensure we are not pulling in a different protobuf library on the
+        # system.
+        self.assertTrue(_message._IS_UPB)
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/python/protobuf.c b/python/protobuf.c
index 62398cf471..eeea3b3e6a 100644
--- a/python/protobuf.c
+++ b/python/protobuf.c
@@ -27,10 +27,11 @@
 
 #include "protobuf.h"
 
+#include "descriptor.h"
 #include "descriptor_pool.h"
 
-static void PyUpb_ModuleDealloc(void *_s) {
-  PyUpb_ModuleState *s = _s;
+static void PyUpb_ModuleDealloc(void *module) {
+  PyUpb_ModuleState *s = PyModule_GetState(module);
   upb_arena_free(s->obj_cache_arena);
 }
 
@@ -114,10 +115,14 @@ PyMODINIT_FUNC PyInit__message(void) {
   state->obj_cache_arena = upb_arena_new();
   upb_inttable_init(&state->obj_cache, state->obj_cache_arena);
 
-  if (!PyUpb_InitDescriptorPool(m)) {
+  if (!PyUpb_InitDescriptorPool(m) || !PyUpb_InitDescriptor(m)) {
     Py_DECREF(m);
     return NULL;
   }
 
+  // Temporary: an cookie we can use in the tests to ensure we are testing upb
+  // and not another protobuf library on the system.
+  PyModule_AddObject(m, "_IS_UPB", Py_True);
+
   return m;
 }