Add framework for AST transformations of input code.

IPython/core/interactiveshell.py

@@ -199,6 +199,13 b' class InteractiveShell(SingletonConfigurable):'
     """An enhanced, interactive shell for Python."""
 
     _instance = None
+    
+    ast_transformers = List([], config=True, help=
+        """
+        A list of ast.NodeTransformer subclass instances, which will be applied
+        to user input before code is run.
+        """
+    )
 
     autocall = Enum((0,1,2), default_value=0, config=True, help=
         """
@@ -2630,6 +2637,8 b' class InteractiveShell(SingletonConfigurable):'
                             self.execution_count += 1
                         return None
                     
+                    code_ast = self.transform_ast(code_ast)
+                    
                     interactivity = "none" if silent else self.ast_node_interactivity
                     self.run_ast_nodes(code_ast.body, cell_name,
                                        interactivity=interactivity)
@@ -2662,6 +2671,31 b' class InteractiveShell(SingletonConfigurable):'
             self.history_manager.store_output(self.execution_count)
             # Each cell is a *single* input, regardless of how many lines it has
             self.execution_count += 1
+    
+    def transform_ast(self, node):
+        """Apply the AST transformations from self.ast_transformers
+        
+        Parameters
+        ----------
+        node : ast.Node
+          The root node to be transformed. Typically called with the ast.Module
+          produced by parsing user input.
+        
+        Returns
+        -------
+        An ast.Node corresponding to the node it was called with. Note that it
+        may also modify the passed object, so don't rely on references to the
+        original AST.
+        """
+        for transformer in self.ast_transformers:
+            try:
+                node = transformer.visit(node)
+            except Exception:
+                warn("AST transformer %r threw an error. It will be unregistered.")
+                self.ast_transformers.remove(transformer)
+        
+        return node
+                
 
     def run_ast_nodes(self, nodelist, cell_name, interactivity='last_expr'):
         """Run a sequence of AST nodes. The execution mode depends on the

IPython/core/tests/test_interactiveshell.py

@@ -20,6 +20,7 b' Authors'
 # Imports
 #-----------------------------------------------------------------------------
 # stdlib
+import ast
 import os
 import shutil
 import sys
@@ -414,6 +415,28 b' class TestModules(unittest.TestCase, tt.TempFileMixin):'
         out = "False\nFalse\nFalse\n"
         tt.ipexec_validate(self.fname, out)
 
+class Negator(ast.NodeTransformer):
+    """Negates all number literals in an AST."""
+    def visit_Num(self, node):
+        node.n = -node.n
+        return node
+
+class TestAstTransform(unittest.TestCase):
+    def setUp(self):
+        self.negator = Negator()
+        ip.ast_transformers.append(self.negator)
+    
+    def tearDown(self):
+        ip.ast_transformers.remove(self.negator)
+    
+    def test_run_cell(self):
+        with tt.AssertPrints('-34'):
+            ip.run_cell('print (12 + 22)')
+        
+        # A named reference to a number shouldn't be transformed.
+        ip.user_ns['n'] = 55
+        with tt.AssertNotPrints('-55'):
+            ip.run_cell('print (n)')
 
 def test__IPYTHON__():
     # This shouldn't raise a NameError, that's all