Make the TerminalInteractiveShell class configurable.

Should allow custom frontend in separate packages.

IPython/terminal/ipapp.py

@@ -33,7 +33,7 b' from IPython.extensions.storemagic import StoreMagics'
 from .interactiveshell import TerminalInteractiveShell
 from IPython.paths import get_ipython_dir
 from traitlets import (
-    Bool, List, Dict, default, observe,
+    Bool, List, Dict, default, observe, Type
 )
 
 #-----------------------------------------------------------------------------
@@ -183,6 +183,13 b' class TerminalIPythonApp(BaseIPythonApplication, InteractiveShellApp):'
     flags = Dict(flags)
     aliases = Dict(aliases)
     classes = List()
+
+    interactive_shell_class = Type(
+        klass=object,   # use default_value otherwise which only allow subclasses.
+        default_value=TerminalInteractiveShell,
+        help="Class to use to instantiate the TerminalInteractiveShell object. Useful for custom Frontends"
+    ).tag(config=True)
+
     @default('classes')
     def _classes_default(self):
         """This has to be in a method, for TerminalIPythonApp to be available."""
@@ -318,7 +325,7 b' class TerminalIPythonApp(BaseIPythonApplication, InteractiveShellApp):'
         # shell.display_banner should always be False for the terminal
         # based app, because we call shell.show_banner() by hand below
         # so the banner shows *before* all extension loading stuff.
-        self.shell = TerminalInteractiveShell.instance(parent=self,
+        self.shell = self.interactive_shell_class.instance(parent=self,
                         profile_dir=self.profile_dir,
                         ipython_dir=self.ipython_dir, user_ns=self.user_ns)
         self.shell.configurables.append(self)

docs/source/whatsnew/development.rst

@@ -118,6 +118,15 b' of these as well to improve user experience with better error messages and'
 hints.
 
 
+Configurable TerminalInteractiveShell
+-------------------------------------
+
+IPython gained a new ``c.TerminalIPythonApp.interactive_shell_class`` option
+that allow to customize the class used to start the terminal frontend. This
+should allow user to use custom interfaces, like reviving the former readline
+interface which is now a separate package not maintained by the core team.
+
+
 Miscs improvements
 ------------------