Separate InteractiveShell.input_splitter into two instances.

* input_splitter is used to detect when a cell is complete
* input_transformer_manager transforms completed cells

Closes gh-3178

IPython/core/interactiveshell.py

@@ -283,10 +283,16 b' class InteractiveShell(SingletonConfigurable):'
     filename = Unicode("<ipython console>")
     ipython_dir= Unicode('', config=True) # Set to get_ipython_dir() in __init__
 
-    # Input splitter, to split entire cells of input into either individual
-    # interactive statements or whole blocks.
+    # Input splitter, to transform input line by line and detect when a block
+    # is ready to be executed.
     input_splitter = Instance('IPython.core.inputsplitter.IPythonInputSplitter',
-                              (), {})
+                              (), {'line_input_checker': True})
+    
+    # This InputSplitter instance is used to transform completed cells before
+    # running them. It allows cell magics to contain blank lines.
+    input_transformer_manager = Instance('IPython.core.inputsplitter.IPythonInputSplitter',
+                                         (), {'line_input_checker': False})
+    
     logstart = CBool(False, config=True, help=
         """
         Start logging to the default log file.
@@ -2581,10 +2587,9 b' class InteractiveShell(SingletonConfigurable):'
         if silent:
             store_history = False
 
-        self.input_splitter.push(raw_cell)
+        self.input_transformer_manager.push(raw_cell)
+        cell = self.input_transformer_manager.source_reset()
 
-        cell = self.input_splitter.source_reset()
-        
         # Our own compiler remembers the __future__ environment. If we want to
         # run code with a separate __future__ environment, use the default
         # compiler

docs/source/config/inputtransforms.txt

@@ -35,6 +35,14 b' in attributes of :class:`~IPython.core.inputsplitter.IPythonInputSplitter`:'
   passed to these, but note that function and class *definitions* are still a
   series of separate statements. IPython does not use any of these by default.
 
+An InteractiveShell instance actually has two
+:class:`~IPython.core.inputsplitter.IPythonInputSplitter` instances, as the
+attributes :attr:`~IPython.core.interactiveshell.InteractiveShell.input_splitter`,
+to tell when a block of input is complete, and
+:attr:`~IPython.core.interactiveshell.InteractiveShell.input_transformer_manager`,
+to transform complete cells. If you add a transformer, you should make sure that
+it gets added to both.
+
 Stateless transformations
 -------------------------