Make running PYTHONSTARTUP optional

It makes sense either to have PYTHONSTARTUP set up things for all
interactive Python sessions, or to bring some convenience features to
the default Python shell. We used to ignore it, but IPython 1.x executes
the PYTHONSTARTUP file. In conversation with @minrk, we felt there
should be a config option not to run it, but running it should remain
the default.

IPython/core/shellapp.py

@@ -172,6 +172,10 b' class InteractiveShellApp(Configurable):'
     exec_files = List(Unicode, config=True,
         help="""List of files to run at IPython startup."""
     )
+    exec_PYTHONSTARTUP = Bool(True, config=True,
+        help="""Run the file referenced by the PYTHONSTARTUP environment
+        variable at IPython startup."""
+    )
     file_to_run = Unicode('', config=True,
         help="""A file to be run""")
 
@@ -354,8 +358,9 b' class InteractiveShellApp(Configurable):'
         """Run files from profile startup directory"""
         startup_dir = self.profile_dir.startup_dir
         startup_files = []
-        if os.environ.get('PYTHONSTARTUP', False):
-            startup_files.append(os.environ['PYTHONSTARTUP'])
+        if self.exec_PYTHONSTARTUP:
+            if os.environ.get('PYTHONSTARTUP', False):
+                startup_files.append(os.environ['PYTHONSTARTUP'])
         startup_files += glob.glob(os.path.join(startup_dir, '*.py'))
         startup_files += glob.glob(os.path.join(startup_dir, '*.ipy'))
         if not startup_files:

docs/source/interactive/reference.rst

@@ -761,10 +761,10 b" won't work::"
 IPython as your default Python environment
 ==========================================
 
-Python honors the environment variable PYTHONSTARTUP and will execute at
-startup the file referenced by this variable. If you put the following code at
-the end of that file, then IPython will be your working environment anytime you
-start Python::
+Python honors the environment variable :envvar:`PYTHONSTARTUP` and will
+execute at startup the file referenced by this variable. If you put the
+following code at the end of that file, then IPython will be your working
+environment anytime you start Python::
 
     from IPython.frontend.terminal.ipapp import launch_new_instance
     launch_new_instance()
@@ -774,6 +774,11 b' The ``raise SystemExit`` is needed to exit Python when'
 it finishes, otherwise you'll be back at the normal Python '>>>'
 prompt.
 
+You'll also need to set the config option
+``InteractiveShellApp.exec_PYTHONSTARTUP = False``, otherwise IPython
+will try to run :envvar:`PYTHONSTARTUP` again, sending it into an
+infinite loop.
+
 This is probably useful to developers who manage multiple Python
 versions and don't want to have correspondingly multiple IPython
 versions. Note that in this mode, there is no way to pass IPython any