Add simple support for running inside a virtualenv.

IPython/core/interactiveshell.py

@@ -411,6 +411,9 b' class InteractiveShell(SingletonConfigurable, Magic):'
         self.init_instance_attrs()
         self.init_environment()
 
+        # Check if we're in a virtualenv, and set up sys.path.
+        self.init_virtualenv()
+
         # Create namespaces (user_ns, user_global_ns, etc.)
         self.init_create_namespaces(user_module, user_ns)
         # This has to be done after init_create_namespaces because it uses
@@ -662,6 +665,37 b' class InteractiveShell(SingletonConfigurable, Magic):'
         except ImportError:
             warn("doctest module does not exist.")
 
+    def init_virtualenv(self):
+        """Add a virtualenv to sys.path so the user can import modules from it.
+        This isn't perfect: it doesn't use the Python interpreter with which the
+        virtualenv was built, and it ignores the --no-site-packages option. A
+        warning will appear suggesting the user installs IPython in the
+        virtualenv, but for many cases, it probably works well enough.
+        
+        Adapted from code snippets online.
+        
+        http://blog.ufsoft.org/2009/1/29/ipython-and-virtualenv
+        """
+        if 'VIRTUAL_ENV' not in os.environ:
+            # Not in a virtualenv
+            return
+        
+        if sys.executable.startswith(os.environ['VIRTUAL_ENV']):
+            # Running properly in the virtualenv, don't need to do anything
+            return
+        
+        warn("Attempting to work in a virtualenv. If you encounter problems, please "
+             "install IPython inside the virtualenv.\n")
+        if sys.platform == "win32":
+            virtual_env = os.path.join(os.environ['VIRTUAL_ENV'], 'Lib', 'site-packages') 
+        else:
+            virtual_env = os.path.join(os.environ['VIRTUAL_ENV'], 'lib',
+                       'python%d.%d' % sys.version_info[:2], 'site-packages')
+        
+        import site
+        sys.path.insert(0, virtual_env)
+        site.addsitedir(virtual_env)
+
     #-------------------------------------------------------------------------
     # Things related to injections into the sys module
     #-------------------------------------------------------------------------