actor: support custom Python loader hooks

Fixes #750

direct/src/actor/Actor.py

@@ -5,6 +5,7 @@ __all__ = ['Actor']
 from panda3d.core import *
 from panda3d.core import Loader as PandaLoader
 from direct.showbase.DirectObject import DirectObject
+from direct.showbase.Loader import Loader
 from direct.directnotify import DirectNotifyGlobal
 
 
@@ -1886,6 +1887,9 @@ class Actor(DirectObject, NodePath):
                 else:
                     loaderOptions.setFlags(loaderOptions.getFlags() | LoaderOptions.LFReportErrors)
 
+            # Ensure that custom Python loader hooks are initialized.
+            Loader._loadPythonFileTypes()
+
             # Pass loaderOptions to specify that we want to
             # get the skeleton model.  This only matters to model
             # files (like .mb) for which we can choose to extract

direct/src/showbase/Loader.py

@@ -22,6 +22,8 @@ class Loader(DirectObject):
     notify = directNotify.newCategory("Loader")
     loaderIndex = 0
 
+    _loadedPythonFileTypes = False
+
     class Callback:
         """Returned by loadModel when used asynchronously.  This class is
         modelled after Future, and can be awaited."""
@@ -149,8 +151,7 @@ class Loader(DirectObject):
         Loader.loaderIndex += 1
         self.accept(self.hook, self.__gotAsyncObject)
 
-        if ConfigVariableBool('loader-support-entry-points', True):
-            self._loadPythonFileTypes()
+        self._loadPythonFileTypes()
 
     def destroy(self):
         self.ignore(self.hook)
@@ -158,7 +159,14 @@ class Loader(DirectObject):
         del self.base
         del self.loader
 
-    def _loadPythonFileTypes(self):
+    @classmethod
+    def _loadPythonFileTypes(cls):
+        if cls._loadedPythonFileTypes:
+            return
+
+        if not ConfigVariableBool('loader-support-entry-points', True):
+            return
+
         import importlib
         try:
             pkg_resources = importlib.import_module('pkg_resources')
@@ -171,6 +179,8 @@ class Loader(DirectObject):
             for entry_point in pkg_resources.iter_entry_points('panda3d.loaders'):
                 registry.register_deferred_type(entry_point)
 
+            cls._loadedPythonFileTypes = True
+
     # model loading funcs
     def loadModel(self, modelPath, loaderOptions = None, noCache = None,
                   allowInstance = False, okMissing = None,