Allow configuring a function to handle return in the terminal

As suggested on gh-10498

IPython/terminal/interactiveshell.py

@@ -10,7 +10,10 b' from IPython.utils import io'
 from IPython.utils.py3compat import cast_unicode_py2, input
 from IPython.utils.terminal import toggle_set_term_title, set_term_title
 from IPython.utils.process import abbrev_cwd
-from traitlets import Bool, Unicode, Dict, Integer, observe, Instance, Type, default, Enum, Union
+from traitlets import (
+    Bool, Unicode, Dict, Integer, observe, Instance, Type, default, Enum, Union,
+    Any,
+)
 
 from prompt_toolkit.document import Document
 from prompt_toolkit.enums import DEFAULT_BUFFER, EditingMode
@@ -192,6 +195,12 b' class TerminalInteractiveShell(InteractiveShell):'
              "This is in addition to the F2 binding, which is always enabled."
     ).tag(config=True)
 
+    handle_return = Any(None,
+        help="Provide an alternative handler to be called when the user presses "
+             "Return. This is an advanced option intended for debugging, which "
+             "may be changed or removed in later releases."
+    ).tag(config=True)
+
     @observe('term_title')
     def init_term_title(self, change=None):
         # Enable or disable the terminal title.

IPython/terminal/shortcuts.py

@@ -31,12 +31,17 b' def register_ipython_shortcuts(registry, shell):'
     """Set up the prompt_toolkit keyboard shortcuts for IPython"""
     insert_mode = ViInsertMode() | EmacsInsertMode()
 
+    if getattr(shell, 'handle_return', None):
+        return_handler = shell.handle_return(shell)
+    else:
+        return_handler = newline_or_execute_outer(shell)
+
     # Ctrl+J == Enter, seemingly
     registry.add_binding(Keys.ControlJ,
                          filter=(HasFocus(DEFAULT_BUFFER)
                                  & ~HasSelection()
                                  & insert_mode
-                        ))(newline_or_execute_outer(shell))
+                        ))(return_handler)
 
     registry.add_binding(Keys.ControlBackslash)(force_exit)