[pygobject] Explicitly check if an override exists instead of ImportError

[Garrett Regier <gregier src gnome org>]

commit 79cf1f70d247b5a4d33d1e60107e47903ca76055
Author: Garrett Regier <garrett regier riftio com>
Date:   Mon May 18 02:32:18 2015 -0700

    Explicitly check if an override exists instead of ImportError
    
    If an override depended on another module and it did not
    exist then the raised ImportError was consumed and assumed
    to mean that the override did not exist. This makes it
    difficult to diagnose issues with overrides.
    
    This uses pkgutil.get_loader() as it is the easier way to
    determine if a module exists in both Python 2 and 3
    and avoid deprecated functions.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=749532

 gi/overrides/__init__.py |   17 +++++++++++++++--
 1 files changed, 15 insertions(+), 2 deletions(-)
---
diff --git a/gi/overrides/__init__.py b/gi/overrides/__init__.py
index bc915b2..942e6ed 100644
--- a/gi/overrides/__init__.py
+++ b/gi/overrides/__init__.py
@@ -2,6 +2,7 @@ import types
 import warnings
 import importlib
 import sys
+from pkgutil import get_loader
 
 from gi import PyGIDeprecationWarning
 from gi._gi import CallableInfo
@@ -107,10 +108,22 @@ def load_overrides(introspection_module):
     modules[namespace] = proxy
 
     try:
+        override_package_name = 'gi.overrides.' + namespace
+
+        # http://bugs.python.org/issue14710
         try:
-            override_mod = importlib.import_module('gi.overrides.' + namespace)
-        except ImportError:
+            override_loader = get_loader(override_package_name)
+
+        except AttributeError:
+            override_loader = None
+
+        # Avoid checking for an ImportError, an override might
+        # depend on a missing module thus causing an ImportError
+        if override_loader is None:
             return introspection_module
+
+        override_mod = importlib.import_module(override_package_name)
+
     finally:
         del modules[namespace]
         del sys.modules[module_key]