diff --git a/ceilometer/agent/plugin_base.py b/ceilometer/agent/plugin_base.py
index 902ed3b45..ebc44c5d1 100644
--- a/ceilometer/agent/plugin_base.py
+++ b/ceilometer/agent/plugin_base.py
@@ -72,7 +72,7 @@ def check_keystone(service_type=None):
                 keystone = _get_keystone()
             if isinstance(keystone, Exception):
                 LOG.error(_('Skip due to keystone error %s'),
-                          str(keystone) if keystone else '')
+                          keystone if keystone else '')
                 return iter([])
             elif service_type:
                 endpoints = keystone.service_catalog.get_endpoints(
diff --git a/ceilometer/collector.py b/ceilometer/collector.py
index d715f72f7..756b4ee43 100644
--- a/ceilometer/collector.py
+++ b/ceilometer/collector.py
@@ -99,10 +99,10 @@ class CollectorService(os_service.Service):
             try:
                 sample = msgpack.loads(data, encoding='utf-8')
             except Exception:
-                LOG.warn(_("UDP: Cannot decode data sent by %s"), str(source))
+                LOG.warn(_("UDP: Cannot decode data sent by %s"), source)
             else:
                 try:
-                    LOG.debug(_("UDP: Storing %s"), str(sample))
+                    LOG.debug(_("UDP: Storing %s"), sample)
                     self.dispatcher_manager.map_method('record_metering_data',
                                                        sample)
                 except Exception:
diff --git a/ceilometer/transformer/arithmetic.py b/ceilometer/transformer/arithmetic.py
index 2dbc5c931..37fa692cc 100644
--- a/ceilometer/transformer/arithmetic.py
+++ b/ceilometer/transformer/arithmetic.py
@@ -93,7 +93,7 @@ class ArithmeticTransformer(transformer.TransformerBase):
             )
         except Exception as e:
             LOG.warn(_('Unable to evaluate expression %(expr)s: %(exc)s'),
-                     {'expr': self.expr, 'exc': str(e)})
+                     {'expr': self.expr, 'exc': e})
 
     def handle_sample(self, context, _sample):
         self._update_cache(_sample)