Fix lazy loading of algorithm's runner

The runner associated with an algorithm should only be created once.

This patch fixes that.

beat/backend/python/executor.py

@@ -92,7 +92,7 @@ class Executor(object):
             self.data = simplejson.loads(f.read().decode('utf-8'))
 
         self.prefix = os.path.join(directory, 'prefix')
-        self.runner = None
+        self._runner = None
 
         # Temporary caches, if the user has not set them, for performance
         database_cache = database_cache if database_cache is not None else {}
@@ -134,10 +134,21 @@ class Executor(object):
             )
 
 
+    @property
+    def runner(self):
+        """Returns the algorithm runner
+
+        This property allows for lazy loading of the runner
+        """
+
+        if self._runner is None:
+            self._runner = self.algorithm.runner()
+        return self._runner
+
+
     def setup(self):
         """Sets up the algorithm to start processing"""
 
-        self.runner = self.algorithm.runner()
         retval = self.runner.setup(self.data['parameters'])
         logger.debug("User algorithm is setup")
         return retval
@@ -146,7 +157,6 @@ class Executor(object):
     def prepare(self):
         """Prepare the algorithm"""
 
-        self.runner = self.algorithm.runner()
         retval = self.runner.prepare(self.data_loaders)
         logger.debug("User algorithm is prepared")
         return retval