Limit special-casing of _ variable to doctests.

In doctests, _ *must* be special-cased and removed from the user's
namespace because otherwise Python won't set it.  But we were doing
this special-casing unconditionally, making it impossible to write
unittests that checked the _ variable after cell execution.

This commit makes the special-casing of _ apply only in the doctest
execution (where it's needed) and leaves normal unittests alone.

IPython/testing/globalipapp.py

@@ -68,11 +68,19 b' class ipnsdict(dict):'
     This subclass adds a simple checkpointing capability so that when testing
     machinery clears it (we use it as the test execution context), it doesn't
     get completely destroyed.
+
+    In addition, it can handle the presence of the '_' key in a special manner,
+    which is needed because of how Python's doctest machinery operates with
+    '_'.  See constructor and :meth:`update` for details.
     """
     
     def __init__(self,*a):
         dict.__init__(self,*a)
         self._savedict = {}
+        # If this flag is True, the .update() method will unconditionally
+        # remove a key named '_'.  This is so that such a dict can be used as a
+        # namespace in doctests that call '_'.
+        self.protect_underscore = False
         
     def clear(self):
         dict.clear(self)
@@ -86,11 +94,14 b' class ipnsdict(dict):'
         self._checkpoint()
         dict.update(self,other)
 
-        # If '_' is in the namespace, python won't set it when executing code
-        # *in doctests*, and we have multiple doctests that use '_'.  So we
-        # ensure that the namespace is always 'clean' of it before it's used
-        # for test code execution.  Note: this means that outside of doctests,
-        # our own testing 
+        if self.protect_underscore:
+            # If '_' is in the namespace, python won't set it when executing
+            # code *in doctests*, and we have multiple doctests that use '_'.
+            # So we ensure that the namespace is always 'clean' of it before
+            # it's used for test code execution.
+            # This flag is only turned on by the doctest machinery, so that
+            # normal test code can assume the _ key is updated like any other
+            # key and can test for its presence after cell executions.
         self.pop('_',None)
 
         # The builtins namespace must *always* be the real __builtin__ module,

IPython/testing/plugin/ipdoctest.py

@@ -273,6 +273,10 b' class DocTestCase(doctests.DocTestCase):'
             # fills with the necessary info from the module being tested).
             _ip.user_ns.update(self._dt_test.globs)
             self._dt_test.globs = _ip.user_ns
+            # IPython must protect the _ key in the namespace (it can't exist)
+            # so that Python's doctest code sets it naturally, so we enable
+            # this feature of our testing namespace.
+            _ip.user_ns.protect_underscore = True
 
         super(DocTestCase, self).setUp()
 
@@ -282,6 +286,9 b' class DocTestCase(doctests.DocTestCase):'
         # teardown doesn't destroy the ipython namespace
         if isinstance(self._dt_test.examples[0],IPExample):
             self._dt_test.globs = self._dt_test_globs_ori
+            # Restore the behavior of the '_' key in the user namespace to
+            # normal after each doctest, so that unittests behave normally
+            _ip.user_ns.protect_underscore = False
             
         # XXX - fperez: I am not sure if this is truly a bug in nose 0.11, but
         # it does look like one to me: its tearDown method tries to run