py_panda: fix int-to-enum conversion in Python 2

Fixes #498

dtool/src/interrogatedb/py_panda.cxx

@@ -286,6 +286,43 @@ PyObject *_Dtool_Return(PyObject *value) {
 }
 
 #if PY_VERSION_HEX < 0x03040000
+/**
+ * This function converts an int value to the appropriate enum instance.
+ */
+PyObject *Dtool_EnumType_New(PyTypeObject *subtype, PyObject *args, PyObject *kwds) {
+  PyObject *arg;
+  if (!Dtool_ExtractArg(&arg, args, kwds, "value")) {
+    return PyErr_Format(PyExc_TypeError,
+                        "%s() missing 1 required argument: 'value'",
+                        subtype->tp_name);
+  }
+
+  if (Py_TYPE(arg) == subtype) {
+    Py_INCREF(arg);
+    return arg;
+  }
+
+  PyObject *value2member = PyDict_GetItemString(subtype->tp_dict, "_value2member_map_");
+  nassertr_always(value2member != nullptr, nullptr);
+
+  PyObject *member = PyDict_GetItem(value2member, arg);
+  if (member != nullptr) {
+    Py_INCREF(member);
+    return member;
+  }
+
+  PyObject *repr = PyObject_Repr(arg);
+  PyErr_Format(PyExc_ValueError, "%s is not a valid %s",
+#if PY_MAJOR_VERSION >= 3
+               PyUnicode_AS_STRING(repr),
+#else
+               PyString_AS_STRING(repr),
+#endif
+               subtype->tp_name);
+  Py_DECREF(repr);
+  return nullptr;
+}
+
 static PyObject *Dtool_EnumType_Str(PyObject *self) {
   PyObject *name = PyObject_GetAttrString(self, "name");
 #if PY_MAJOR_VERSION >= 3
@@ -337,6 +374,7 @@ PyTypeObject *Dtool_EnumType_Create(const char *name, PyObject *names, const cha
   static PyObject *name_sunder_str;
   static PyObject *value_str;
   static PyObject *value_sunder_str;
+  static PyObject *value2member_map_sunder_str;
   // Emulate something vaguely like the enum module.
   if (enum_class == nullptr) {
 #if PY_MAJOR_VERSION >= 3
@@ -344,11 +382,13 @@ PyTypeObject *Dtool_EnumType_Create(const char *name, PyObject *names, const cha
     value_str = PyUnicode_InternFromString("value");
     name_sunder_str = PyUnicode_InternFromString("_name_");
     value_sunder_str = PyUnicode_InternFromString("_value_");
+    value2member_map_sunder_str = PyUnicode_InternFromString("_value2member_map_");
 #else
     name_str = PyString_InternFromString("name");
     value_str = PyString_InternFromString("value");
     name_sunder_str = PyString_InternFromString("_name_");
     value_sunder_str = PyString_InternFromString("_value_");
+    value2member_map_sunder_str = PyString_InternFromString("_value2member_map_");
 #endif
     PyObject *name_value_tuple = PyTuple_New(4);
     PyTuple_SET_ITEM(name_value_tuple, 0, name_str);
@@ -365,27 +405,39 @@ PyTypeObject *Dtool_EnumType_Create(const char *name, PyObject *names, const cha
     enum_class = PyObject_CallFunction((PyObject *)&PyType_Type, (char *)"s()N", "Enum", slots_dict);
     nassertr(enum_class != nullptr, nullptr);
   }
-  PyObject *result = PyObject_CallFunction((PyObject *)&PyType_Type, (char *)"s(O)N", name, enum_class, PyDict_New());
+
+  // Create a subclass of this generic Enum class we just created.
+  PyObject *value2member = PyDict_New();
+  PyObject *dict = PyDict_New();
+  PyDict_SetItem(dict, value2member_map_sunder_str, value2member);
+  PyObject *result = PyObject_CallFunction((PyObject *)&PyType_Type, (char *)"s(O)N", name, enum_class, dict);
   nassertr(result != nullptr, nullptr);
 
+  ((PyTypeObject *)result)->tp_new = Dtool_EnumType_New;
   ((PyTypeObject *)result)->tp_str = Dtool_EnumType_Str;
   ((PyTypeObject *)result)->tp_repr = Dtool_EnumType_Repr;
 
-  // Copy the names as instances of the above to the class dict.
+  PyObject *empty_tuple = PyTuple_New(0);
+
+  // Copy the names as instances of the above to the class dict, and create a
+  // reverse mapping in the _value2member_map_ dict.
   Py_ssize_t size = PyTuple_GET_SIZE(names);
   for (Py_ssize_t i = 0; i < size; ++i) {
     PyObject *item = PyTuple_GET_ITEM(names, i);
     PyObject *name = PyTuple_GET_ITEM(item, 0);
     PyObject *value = PyTuple_GET_ITEM(item, 1);
-    PyObject *member = _PyObject_CallNoArg(result);
+    PyObject *member = PyType_GenericNew((PyTypeObject *)result, empty_tuple, nullptr);
     PyObject_SetAttr(member, name_str, name);
     PyObject_SetAttr(member, name_sunder_str, name);
     PyObject_SetAttr(member, value_str, value);
     PyObject_SetAttr(member, value_sunder_str, value);
     PyObject_SetAttr(result, name, member);
+    PyDict_SetItem(value2member, value, member);
     Py_DECREF(member);
   }
   Py_DECREF(names);
+  Py_DECREF(value2member);
+  Py_DECREF(empty_tuple);
 #endif
 
   if (module != nullptr) {