Updated comment

IPython/nbconvert/transformers/extractoutput.py

@@ -65,7 +65,12 b' class ExtractOutputTransformer(Transformer):'
 
             #Get the output in data formats that the template is interested in.
             for out_type in self.display_data_priority:
-                if out.hasattr(out_type):
+
+                # Only extract the output if it's data exists as an attribute.
+                # This means things like output_type="stream" where the data
+                # is stored in "text" (instead of "stream"), will not be 
+                # extracted (intentional).
+                if out.hasattr(out_type): 
                     data = out[out_type]
 
                     #Binary files are base64-encoded, SVG is already XML