Add CommandTree.error decorator to set on_error dynamically

discord/app_commands/tree.py

@@ -31,6 +31,7 @@ from typing import (
     Any,
     TYPE_CHECKING,
     Callable,
+    Coroutine,
     Dict,
     Generator,
     Generic,
@@ -66,6 +67,15 @@ if TYPE_CHECKING:
     from ..abc import Snowflake
     from .commands import ContextMenuCallback, CommandCallback, P, T
 
+    ErrorFunc = Callable[
+        [
+            Interaction,
+            Optional[Union[ContextMenu, Command[Any, ..., Any]]],
+            AppCommandError,
+        ],
+        Coroutine[Any, Any, Any],
+    ]
+
 __all__ = ('CommandTree',)
 
 ClientT = TypeVar('ClientT', bound='Client')
@@ -681,6 +691,36 @@ class CommandTree(Generic[ClientT]):
 
         traceback.print_exception(error.__class__, error, error.__traceback__, file=sys.stderr)
 
+    def error(self, coro: ErrorFunc) -> ErrorFunc:
+        """A decorator that registers a coroutine as a local error handler.
+
+        This must match the signature of the :meth:`on_error` callback.
+
+        The error passed will be derived from :exc:`AppCommandError`.
+
+        Parameters
+        -----------
+        coro: :ref:`coroutine <coroutine>`
+            The coroutine to register as the local error handler.
+
+        Raises
+        -------
+        TypeError
+            The coroutine passed is not actually a coroutine or does
+            not match the signature.
+        """
+
+        if not inspect.iscoroutinefunction(coro):
+            raise TypeError('The error handler must be a coroutine.')
+
+        params = inspect.signature(coro).parameters
+        if len(params) != 3:
+            raise TypeError('error handler must have 3 parameters')
+
+        # Type checker doesn't like overriding methods like this
+        self.on_error = coro  # type: ignore
+        return coro
+
     def command(
         self,
         *,