PR: Unassigned allowed commands (#14564)

From git git:

> Modify binding mechanism to allow binding to one of a set of
whitelisted commands, Add five such commands ("beginning_of_buffer",
"end_of_buffer", "end_of_line", "forward_word", "unix_line_discard").

Currently, only commands which are assigned by default are allowed to be
reassigned:

```python
        known_commands = {
            create_identifier(binding.command): binding.command
            for binding in KEY_BINDINGS
        }
        ...
            if command_id not in known_commands:
                ...
                raise ValueError(
                    f"{command_id} is not a known shortcut command."
                    f" Allowed commands are: \n - {allowed_commands}"
                )
```

This PR adds a List UNASSIGNED_ALLOWED_COMMANDS and updates
`known_commands` (now renamed to be more clear as `allowed_commands`):

```python
        allowed_commands.update({
                create_identifier(command): command
                for command in UNASSIGNED_ALLOWED_COMMANDS
        })
```

The five unassigned allowed commands are:


```python
UNASSIGNED_ALLOWED_COMMANDS = [
    nc.beginning_of_buffer,
    nc.end_of_buffer,
    nc.end_of_line,
    nc.forward_word,
    nc.unix_line_discard,
]
```

Motivation tldr: Until now, it is not possible to bind eg the common
keybindng `ctrl+e` to prompt_toolkit's `end_of_line` (because it is not
bound by default).

Full motivation: https://github.com/ipython/ipython/issues/14369

IPython/terminal/interactiveshell.py

@@ -52,6 +52,7 b' from .prompts import Prompts, ClassicPrompts, RichPromptDisplayHook'
 from .ptutils import IPythonPTCompleter, IPythonPTLexer
 from .shortcuts import (
     KEY_BINDINGS,
+    UNASSIGNED_ALLOWED_COMMANDS,
     create_ipython_shortcuts,
     create_identifier,
     RuntimeBinding,
@@ -508,19 +509,25 b' class TerminalInteractiveShell(InteractiveShell):'
         # rebuild the bindings list from scratch
         key_bindings = create_ipython_shortcuts(self)
 
-        # for now we only allow adding shortcuts for commands which are already
-        # registered; this is a security precaution.
-        known_commands = {
+        # for now we only allow adding shortcuts for a specific set of
+        # commands; this is a security precution.
+        allowed_commands = {
             create_identifier(binding.command): binding.command
             for binding in KEY_BINDINGS
         }
+        allowed_commands.update(
+            {
+                create_identifier(command): command
+                for command in UNASSIGNED_ALLOWED_COMMANDS
+            }
+        )
         shortcuts_to_skip = []
         shortcuts_to_add = []
 
         for shortcut in user_shortcuts:
             command_id = shortcut["command"]
-            if command_id not in known_commands:
-                allowed_commands = "\n - ".join(known_commands)
+            if command_id not in allowed_commands:
+                allowed_commands = "\n - ".join(allowed_commands)
                 raise ValueError(
                     f"{command_id} is not a known shortcut command."
                     f" Allowed commands are: \n - {allowed_commands}"
@@ -544,7 +551,7 b' class TerminalInteractiveShell(InteractiveShell):'
             new_keys = shortcut.get("new_keys", None)
             new_filter = shortcut.get("new_filter", None)
 
-            command = known_commands[command_id]
+            command = allowed_commands[command_id]
 
             creating_new = shortcut.get("create", False)
             modifying_existing = not creating_new and (
@@ -586,12 +593,14 b' class TerminalInteractiveShell(InteractiveShell):'
                     RuntimeBinding(
                         command,
                         keys=new_keys or old_keys,
-                        filter=filter_from_string(new_filter)
-                        if new_filter is not None
-                        else (
-                            old_filter
-                            if old_filter is not None
-                            else filter_from_string("always")
+                        filter=(
+                            filter_from_string(new_filter)
+                            if new_filter is not None
+                            else (
+                                old_filter
+                                if old_filter is not None
+                                else filter_from_string("always")
+                            )
                         ),
                     )
                 )

IPython/terminal/shortcuts/__init__.py

@@ -628,3 +628,11 b' KEY_BINDINGS = ['
     *SIMPLE_CONTROL_BINDINGS,
     *ALT_AND_COMOBO_CONTROL_BINDINGS,
 ]
+
+UNASSIGNED_ALLOWED_COMMANDS = [
+    nc.beginning_of_buffer,
+    nc.end_of_buffer,
+    nc.end_of_line,
+    nc.forward_word,
+    nc.unix_line_discard,
+]