only capture the ImportError when importing

If the __init__ function raises a ImportError, it will
also be catched. So we should call the __init__ function
outside the try except clause instead.

Change-Id: Iaa06565087848e2f36e8def8f0922a3ab692b695

oslo_utils/importutils.py

@@ -47,9 +47,10 @@ def import_object_ns(name_space, import_str, *args, **kwargs):
     """
     import_value = "%s.%s" % (name_space, import_str)
     try:
-        return import_class(import_value)(*args, **kwargs)
+        cls = import_class(import_value)
     except ImportError:
-        return import_class(import_str)(*args, **kwargs)
+        cls = import_class(import_str)
+    return cls(*args, **kwargs)
 
 
 def import_module(import_str):

oslo_utils/tests/fake/__init__.py

@@ -21,3 +21,8 @@ class FakeDriver():
 class FakeDriver2():
     def __init__(self, first_arg):
         self.first_arg = first_arg
+
+
+class FakeDriver3():
+    def __init__(self):
+        raise ImportError("ImportError occurs in __init__")

oslo_utils/tests/test_importutils.py

@@ -103,6 +103,10 @@ class ImportUtilsTest(test_base.BaseTestCase):
                                            first_arg=False)
         self.assertEqual(obj.__class__.__name__, 'FakeDriver2')
 
+    def test_import_object_ns_raise_import_error_in_init(self):
+        self.assertRaises(ImportError, importutils.import_object_ns,
+                          'tests2', 'oslo_utils.tests.fake.FakeDriver3')
+
     def test_import_object(self):
         dt = importutils.import_object('datetime.time')
         self.assertTrue(isinstance(dt, sys.modules['datetime'].time))