Add default validation failure expression config

nonebot/command/__init__.py

@@ -3,8 +3,7 @@ import re
 import shlex
 from datetime import datetime
 from typing import (
-    Tuple, Union, Callable, Iterable, Any, Optional, List, Dict,
-    Awaitable
+    Tuple, Union, Callable, Iterable, Any, Optional, List, Dict
 )
 
 from nonebot import NoneBot, permission as perm
@@ -68,19 +67,8 @@ class Command:
             if session.current_arg_filters is not None and \
                     session.current_key is not None:
                 # argument-level filters are given, use them
-                arg = session.current_arg
-                for f in session.current_arg_filters:
-                    try:
-                        res = f(arg)
-                        if isinstance(res, Awaitable):
-                            res = await res
-                        arg = res
-                    except ValidateError as e:
-                        # validation failed
-                        session.pause(e.message)
-
-                # passed all filters
-                session.state[session.current_key] = arg
+                await argfilter.run_arg_filters(
+                    session, session.current_arg_filters)
             else:
                 # fallback to command-level args_parser_func
                 if self.args_parser_func:

nonebot/command/argfilter/__init__.py

@@ -1,4 +1,9 @@
-from typing import Callable, Any, Awaitable, Union
+from typing import Callable, Any, Awaitable, Union, List, TYPE_CHECKING
+
+from nonebot.helpers import render_expression
+
+if TYPE_CHECKING:
+    from nonebot.command import CommandSession
 
 ArgFilter_T = Callable[[Any], Union[Any, Awaitable[Any]]]
 
@@ -6,3 +11,25 @@ ArgFilter_T = Callable[[Any], Union[Any, Awaitable[Any]]]
 class ValidateError(ValueError):
     def __init__(self, message=None):
         self.message = message
+
+
+async def run_arg_filters(session: 'CommandSession',
+                          arg_filters: List[ArgFilter_T]) -> None:
+    arg = session.current_arg
+    for f in arg_filters:
+        try:
+            res = f(arg)
+            if isinstance(res, Awaitable):
+                res = await res
+            arg = res
+        except ValidateError as e:
+            # validation failed
+            failure_message = e.message
+            if failure_message is None:
+                failure_message = render_expression(
+                    session.bot.config.DEFAULT_VALIDATION_FAILURE_EXPRESSION
+                )
+            session.pause(failure_message)
+
+    # passed all filters
+    session.state[session.current_key] = arg