simplify event source retrieval

@@ -22,10 +22,10 @@ import sys
 
 import kappa.function
 import kappa.restapi
-import kappa.event_source.dynamodb_stream as dynamodb_stream
-import kappa.event_source.kinesis as kinesis
-import kappa.event_source.s3 as s3
-import kappa.event_source.sns as sns
+import kappa.event_source.dynamodb_stream
+import kappa.event_source.kinesis
+import kappa.event_source.s3
+import kappa.event_source.sns
 import kappa.policy
 import kappa.role
 import kappa.awsclient
@@ -168,27 +168,27 @@ class Context(object):
 
     def _create_event_sources(self):
         env_cfg = self.config['environments'][self.environment]
-        if 'event_sources' in env_cfg:
+        if 'event_sources' not in env_cfg:
+            return
+
         event_sources = env_cfg.get('event_sources', {})
         if not event_sources:
             return
+
+        event_source_map = {
+            'dynamodb': kappa.event_source.dynamodb_stream.DynamoDBStreamEventSource,
+            'kinesis': kappa.event_source.kinesis.KinesisEventSource,
+            's3': kappa.event_source.s3.S3EventSource,
+            'sns': kappa.event_source.sns.SNSEventSource,
+        }
         for event_source_cfg in event_sources:
             _, _, svc, _ = event_source_cfg['arn'].split(':', 3)
-                if svc == 'kinesis':
-                    self.event_sources.append(
-                        kinesis.KinesisEventSource(self, event_source_cfg))
-                elif svc == 's3':
+            event_source = event_source_map.get(svc, None)
+            if not event_source:
+                raise ValueError('Unknown event source: {0}'.format(
+                    event_source_cfg['arn']))
             self.event_sources.append(
-                        s3.S3EventSource(self, event_source_cfg))
-                elif svc == 'sns':
-                    self.event_sources.append(
-                        sns.SNSEventSource(self, event_source_cfg))
-                elif svc == 'dynamodb':
-                    self.event_sources.append(
-                        dynamodb_stream.DynamoDBStreamEventSource(self, event_source_cfg))
-                else:
-                    msg = 'Unknown event source: %s' % event_source_cfg['arn']
-                    raise ValueError(msg)
+                event_source(self, event_source_cfg))
 
     def add_event_sources(self):
         for event_source in self.event_sources: