From 4b6047edc2da2c45bbd3513886fcc5bda9a57597 2009-11-09 00:55:44
From: Brian Granger <ellisonbg@gmail.com>
Date: 2009-11-09 00:55:44
Subject: [PATCH] Fixing small bug in activate.

If the parallelmagic extension has not been loaded, activate would
get an empty list from get_component. Now get_component returns None
in that case and activate checks for None and prints a message to
the user to load the extension.

---

diff --git a/IPython/core/iplib.py b/IPython/core/iplib.py
index be513d3..81a5970 100644
--- a/IPython/core/iplib.py
+++ b/IPython/core/iplib.py
@@ -2294,6 +2294,8 @@ class InteractiveShell(Component, Magic):
     def get_component(self, name=None, klass=None):
         """Fetch a component by name and klass in my tree."""
         c = Component.get_instances(root=self, name=name, klass=klass)
+        if len(c) == 0:
+            return None
         if len(c) == 1:
             return c[0]
         else:
diff --git a/IPython/kernel/multiengineclient.py b/IPython/kernel/multiengineclient.py
index 256c174..ccdb51b 100644
--- a/IPython/kernel/multiengineclient.py
+++ b/IPython/kernel/multiengineclient.py
@@ -269,8 +269,12 @@ class InteractiveMultiEngineClient(object):
             print "The IPython parallel magics (%result, %px, %autopx) only work within IPython."
         else:
             pmagic = ip.get_component('parallel_magic')
-            pmagic.active_multiengine_client = self
-                    
+            if pmagic is not None:
+                pmagic.active_multiengine_client = self
+            else:
+                print "You must first load the parallelmagic extension " \
+                      "by doing '%load_ext parallelmagic'"
+
     def __setitem__(self, key, value):
         """Add a dictionary interface for pushing/pulling.