Simplify

IPython/nbconvert/nbconvertapp.py

@@ -160,8 +160,9 b' class NbConvertApp(BaseIPythonApplication):'
     writer_factory = Type()
 
     def _writer_class_changed(self, name, old, new):
-        if new.lower() in self.writer_aliases:
-            new = self.writer_aliases[new.lower()]
+        new = new.lower()
+        if new in self.writer_aliases:
+            new = self.writer_aliases[new]
         self.writer_factory = import_item(new)
 
     # Post-processor specific variables
@@ -177,8 +178,9 b' class NbConvertApp(BaseIPythonApplication):'
     post_processor_factory = Type()
 
     def _post_processor_class_changed(self, name, old, new):
-        if new.lower() in self.post_processor_aliases:
-            new = self.post_processor_aliases[new.lower()]
+        new = new.lower()
+        if new in self.post_processor_aliases:
+            new = self.post_processor_aliases[new]
         if new:
             self.post_processor_factory = import_item(new)