Merge pull request #1059 from fperez/__IPYTHON__

Switch to simple `__IPYTHON__` global to indicate an IPython Shell object has been created.  Note that this does *not* try to track whether user code is being executed by ipython via %run, nor whether the Shell object itself is running an interactive event loop or not.

So the answer for how people should query whether IPython objects are active is now simply

```
try:
  __IPYTHON__
except NameError:
  print 'not in IPython'
```

We do not attempt to track activity levels anymore, as we realized that logic was ultimately to brittle and error prone to be of any real use.

IPython/core/builtin_trap.py

@@ -88,17 +88,12 b' class BuiltinTrap(Configurable):'
             self._orig_builtins[key] = orig
             bdict[key] = value
 
-    def remove_builtin(self, key):
+    def remove_builtin(self, key, orig):
         """Remove an added builtin and re-set the original."""
-        try:
-            orig = self._orig_builtins.pop(key)
-        except KeyError:
-            pass
+        if orig is BuiltinUndefined:
+            del __builtin__.__dict__[key]
         else:
-            if orig is BuiltinUndefined:
-                del __builtin__.__dict__[key]
-            else:
-                __builtin__.__dict__[key] = orig
+            __builtin__.__dict__[key] = orig
 
     def activate(self):
         """Store ipython references in the __builtin__ namespace."""
@@ -107,22 +102,11 b' class BuiltinTrap(Configurable):'
         for name, func in self.auto_builtins.iteritems():
             add_builtin(name, func)
 
-        # Keep in the builtins a flag for when IPython is active.  We set it
-        # with setdefault so that multiple nested IPythons don't clobber one
-        # another.
-        __builtin__.__dict__.setdefault('__IPYTHON__active', 0)
-
     def deactivate(self):
         """Remove any builtins which might have been added by add_builtins, or
         restore overwritten ones to their previous values."""
-        # Note: must iterate over a static keys() list because we'll be
-        # mutating the dict itself
         remove_builtin = self.remove_builtin
-        for key in self._orig_builtins.keys():
-            remove_builtin(key)
+        for key, val in self._orig_builtins.iteritems():
+            remove_builtin(key, val)
         self._orig_builtins.clear()
         self._builtins_added = False
-        try:
-            del __builtin__.__dict__['__IPYTHON__active']
-        except KeyError:
-            pass

IPython/core/interactiveshell.py

@@ -605,6 +605,20 b' class InteractiveShell(SingletonConfigurable, Magic):'
             self.magic_logstart()
 
     def init_builtins(self):
+        # A single, static flag that we set to True.  Its presence indicates
+        # that an IPython shell has been created, and we make no attempts at
+        # removing on exit or representing the existence of more than one
+        # IPython at a time.
+        builtin_mod.__dict__['__IPYTHON__'] = True
+
+        # In 0.11 we introduced '__IPYTHON__active' as an integer we'd try to
+        # manage on enter/exit, but with all our shells it's virtually
+        # impossible to get all the cases right.  We're leaving the name in for
+        # those who adapted their codes to check for this flag, but will
+        # eventually remove it after a few more releases.
+        builtin_mod.__dict__['__IPYTHON__active'] = \
+                                          'Deprecated, check for __IPYTHON__'
+
         self.builtin_trap = BuiltinTrap(shell=self)
 
     def init_inspector(self):

IPython/core/tests/test_interactiveshell.py

@@ -271,3 +271,8 b' class TestSystemRaw(unittest.TestCase):'
         """
         cmd = ur'''python -c "'åäö'"   '''
         _ip.shell.system_raw(cmd)
+
+
+def test__IPYTHON__():
+    # This shouldn't raise a NameError, that's all
+    __IPYTHON__

IPython/frontend/terminal/interactiveshell.py

@@ -341,9 +341,6 b' class TerminalInteractiveShell(InteractiveShell):'
 
         more = False
 
-        # Mark activity in the builtins
-        __builtin__.__dict__['__IPYTHON__active'] += 1
-
         if self.has_readline:
             self.readline_startup_hook(self.pre_readline)
             hlen_b4_cell = self.readline.get_current_history_length()
@@ -413,9 +410,6 b' class TerminalInteractiveShell(InteractiveShell):'
                     hlen_b4_cell = \
                         self._replace_rlhist_multiline(source_raw, hlen_b4_cell)
 
-        # We are off again...
-        __builtin__.__dict__['__IPYTHON__active'] -= 1
-
         # Turn off the exit flag, so the mainloop can be restarted if desired
         self.exit_now = False