[commands] Add support for FlagConverter in hybrid commands

This works by unpacking and repacking the flag arguments in a flag.
If an unsupported type annotation is found then it will error at
definition time.

discord/ext/commands/flags.py

@@ -28,7 +28,7 @@ import inspect
 import re
 import sys
 from dataclasses import dataclass, field
-from typing import TYPE_CHECKING, Any, Dict, Iterator, List, Literal, Optional, Pattern, Set, Tuple, Union
+from typing import TYPE_CHECKING, Any, Dict, Iterator, List, Literal, Optional, Pattern, Set, Tuple, Type, Union
 
 from discord.utils import MISSING, maybe_coroutine, resolve_annotation
 
@@ -44,7 +44,7 @@ __all__ = (
 
 
 if TYPE_CHECKING:
-    from typing_extensions import Self
+    from typing_extensions import Self, TypeGuard
 
     from ._types import BotT
     from .context import Context
@@ -76,6 +76,9 @@ class Flag:
         A negative value indicates an unlimited amount of arguments.
     override: :class:`bool`
         Whether multiple given values overrides the previous value.
+    description: :class:`str`
+        The description of the flag. Shown for hybrid commands when they're
+        used as application commands.
     """
 
     name: str = MISSING
@@ -85,6 +88,7 @@ class Flag:
     default: Any = MISSING
     max_args: int = MISSING
     override: bool = MISSING
+    description: str = MISSING
     cast_to_dict: bool = False
 
     @property
@@ -104,6 +108,7 @@ def flag(
     max_args: int = MISSING,
     override: bool = MISSING,
     converter: Any = MISSING,
+    description: str = MISSING,
 ) -> Any:
     """Override default functionality and parameters of the underlying :class:`FlagConverter`
     class attributes.
@@ -128,8 +133,23 @@ def flag(
     converter: Any
         The converter to use for this flag. This replaces the annotation at
         runtime which is transparent to type checkers.
+    description: :class:`str`
+        The description of the flag. Shown for hybrid commands when they're
+        used as application commands.
     """
-    return Flag(name=name, aliases=aliases, default=default, max_args=max_args, override=override, annotation=converter)
+    return Flag(
+        name=name,
+        aliases=aliases,
+        default=default,
+        max_args=max_args,
+        override=override,
+        annotation=converter,
+        description=description,
+    )
+
+
+def is_flag(obj: Any) -> TypeGuard[Type[FlagConverter]]:
+    return hasattr(obj, '__commands_is_flag__')
 
 
 def validate_flag_name(name: str, forbidden: Set[str]) -> None:

discord/ext/commands/hybrid.py

@@ -31,6 +31,7 @@ from typing import (
     ClassVar,
     Dict,
     List,
+    Tuple,
     Type,
     TypeVar,
     Union,
@@ -45,6 +46,7 @@ from .core import Command, Group
 from .errors import BadArgument, CommandRegistrationError, CommandError, HybridCommandError, ConversionError
 from .converter import Converter, Range, Greedy, run_converters
 from .parameters import Parameter
+from .flags import is_flag, FlagConverter
 from .cog import Cog
 from .view import StringView
 
@@ -163,14 +165,13 @@ def make_greedy_transformer(converter: Any, parameter: Parameter) -> Type[app_co
     return type('GreedyTransformer', (app_commands.Transformer,), {'transform': classmethod(transform)})
 
 
-def replace_parameters(parameters: Dict[str, Parameter], signature: inspect.Signature) -> List[inspect.Parameter]:
-    # Need to convert commands.Parameter back to inspect.Parameter so this will be a bit ugly
-    params = signature.parameters.copy()
-    for name, parameter in parameters.items():
-        converter = parameter.converter
-        # Parameter.converter properly infers from the default and has a str default
-        # This allows the actual signature to inherit this property
-        param = params[name].replace(annotation=converter)
+def replace_parameter(
+    param: inspect.Parameter,
+    converter: Any,
+    callback: Callable[..., Any],
+    original: Parameter,
+    mapping: Dict[str, inspect.Parameter],
+) -> inspect.Parameter:
     try:
         # If it's a supported annotation (i.e. a transformer) just let it pass as-is.
         app_commands.transformers.get_supported_annotation(converter)
@@ -189,7 +190,35 @@ def replace_parameters(parameters: Dict[str, Parameter], signature: inspect.Sign
             if inner is discord.Attachment:
                 raise TypeError('discord.Attachment with Greedy is not supported in hybrid commands')
 
-                param = param.replace(annotation=make_greedy_transformer(inner, parameter))
+            param = param.replace(annotation=make_greedy_transformer(inner, original))
+        elif is_flag(converter):
+            callback.__hybrid_command_flag__ = (param.name, converter)
+            descriptions = {}
+            renames = {}
+            for flag in converter.__commands_flags__.values():
+                name = flag.attribute
+                flag_param = inspect.Parameter(
+                    name=name,
+                    kind=param.kind,
+                    default=flag.default if flag.default is not MISSING else inspect.Parameter.empty,
+                    annotation=flag.annotation,
+                )
+                pseudo = replace_parameter(flag_param, flag.annotation, callback, original, mapping)
+                if name in mapping:
+                    raise TypeError(f'{name!r} flag would shadow a pre-existing parameter')
+                if flag.description is not MISSING:
+                    descriptions[name] = flag.description
+                if flag.name != flag.attribute:
+                    renames[name] = flag.name
+
+                mapping[name] = pseudo
+
+            # Manually call the decorators
+            if descriptions:
+                app_commands.describe(**descriptions)(callback)
+            if renames:
+                app_commands.rename(**renames)(callback)
+
         elif is_converter(converter):
             param = param.replace(annotation=make_converter_transformer(converter))
         elif origin is Union:
@@ -201,12 +230,30 @@ def replace_parameters(parameters: Dict[str, Parameter], signature: inspect.Sign
                     param = param.replace(annotation=Optional[make_converter_transformer(inner)])  # type: ignore
             else:
                 raise
+        elif origin:
+            # Unsupported typing.X annotation e.g. typing.Dict, typing.Tuple, typing.List, etc.
+            raise
         elif callable(converter) and not inspect.isclass(converter):
             param_count = required_pos_arguments(converter)
             if param_count != 1:
                 raise
             param = param.replace(annotation=make_callable_transformer(converter))
 
+    return param
+
+
+def replace_parameters(
+    parameters: Dict[str, Parameter], callback: Callable[..., Any], signature: inspect.Signature
+) -> List[inspect.Parameter]:
+    # Need to convert commands.Parameter back to inspect.Parameter so this will be a bit ugly
+    params = signature.parameters.copy()
+    for name, parameter in parameters.items():
+        converter = parameter.converter
+        # Parameter.converter properly infers from the default and has a str default
+        # This allows the actual signature to inherit this property
+        param = params[name].replace(annotation=converter)
+        param = replace_parameter(param, converter, callback, parameter, params)
+
         if parameter.default is not parameter.empty:
             default = _CallableDefault(parameter.default) if callable(parameter.default) else parameter.default
             param = param.replace(default=default)
@@ -215,6 +262,11 @@ def replace_parameters(parameters: Dict[str, Parameter], signature: inspect.Sign
             # If we're here, then then it hasn't been handled yet so it should be removed completely
             param = param.replace(default=parameter.empty)
 
+        # Flags are flattened out and thus don't get their parameter in the actual mapping
+        if hasattr(converter, '__commands_is_flag__'):
+            del params[name]
+            continue
+
         params[name] = param
 
     return list(params.values())
@@ -223,7 +275,7 @@ def replace_parameters(parameters: Dict[str, Parameter], signature: inspect.Sign
 class HybridAppCommand(discord.app_commands.Command[CogT, P, T]):
     def __init__(self, wrapped: Command[CogT, Any, T]) -> None:
         signature = inspect.signature(wrapped.callback)
-        params = replace_parameters(wrapped.params, signature)
+        params = replace_parameters(wrapped.params, wrapped.callback, signature)
         wrapped.callback.__signature__ = signature.replace(parameters=params)
 
         try:
@@ -237,6 +289,10 @@ class HybridAppCommand(discord.app_commands.Command[CogT, P, T]):
 
         self.wrapped: Command[CogT, Any, T] = wrapped
         self.binding: Optional[CogT] = wrapped.cog
+        # This technically means only one flag converter is supported
+        self.flag_converter: Optional[Tuple[str, Type[FlagConverter]]] = getattr(
+            wrapped.callback, '__hybrid_command_flag__', None
+        )
 
     def _copy_with(self, **kwargs) -> Self:
         copy: Self = super()._copy_with(**kwargs)  # type: ignore
@@ -269,6 +325,19 @@ class HybridAppCommand(discord.app_commands.Command[CogT, P, T]):
             else:
                 transformed_values[param.name] = await param.transform(interaction, value)
 
+        if self.flag_converter is not None:
+            param_name, flag_cls = self.flag_converter
+            flag = flag_cls.__new__(flag_cls)
+            for f in flag_cls.__commands_flags__.values():
+                try:
+                    value = transformed_values.pop(f.attribute)
+                except KeyError:
+                    raise app_commands.CommandSignatureMismatch(self) from None
+                else:
+                    setattr(flag, f.attribute, value)
+
+            transformed_values[param_name] = flag
+
         return transformed_values
 
     async def _check_can_run(self, interaction: discord.Interaction) -> bool:

docs/ext/commands/commands.rst

@@ -876,6 +876,75 @@ A :class:`dict` annotation is functionally equivalent to ``List[Tuple[K, V]]`` e
 given as a :class:`dict` rather than a :class:`list`.
 
 
+Hybrid Command Interaction
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When used as a hybrid command, the parameters are flattened into different parameters for the application command. For example, the following converter:
+
+.. code-block:: python3
+
+    class BanFlags(commands.FlagConverter):
+        member: discord.Member
+        reason: str
+        days: int = 1
+
+
+    @commands.hybrid_command()
+    async def ban(ctx, *, flags: BanFlags):
+        ...
+
+Would be equivalent to an application command defined as this:
+
+.. code-block:: python3
+
+    @commands.hybrid_command()
+    async def ban(ctx, member: discord.Member, reason: str, days: int = 1):
+        ...
+
+This means that decorators that refer to a parameter by name will use the flag name instead:
+
+.. code-block:: python3
+
+    class BanFlags(commands.FlagConverter):
+        member: discord.Member
+        reason: str
+        days: int = 1
+
+
+    @commands.hybrid_command()
+    @app_commands.describe(
+        member='The member to ban',
+        reason='The reason for the ban',
+        days='The number of days worth of messages to delete',
+    )
+    async def ban(ctx, *, flags: BanFlags):
+        ...
+
+For ease of use, the :func:`~ext.commands.flag` function accepts a ``descriptor`` keyword argument to allow you to pass descriptions inline:
+
+.. code-block:: python3
+
+    class BanFlags(commands.FlagConverter):
+        member: discord.Member = commands.flag(description='The member to ban')
+        reason: str = commands.flag(description='The reason for the ban')
+        days: int = 1 = commands.flag(description='The number of days worth of messages to delete')
+
+
+    @commands.hybrid_command()
+    async def ban(ctx, *, flags: BanFlags):
+        ...
+
+
+Note that in hybrid command form, a few annotations are unsupported due to Discord limitations:
+
+- :data:`typing.Tuple`
+- :data:`typing.List`
+- :data:`typing.Dict`
+
+.. note::
+
+    Only one flag converter is supported per hybrid command. Due to the flag converter's way of working, it is unlikely for a user to have two of them in one signature.
+
 .. _ext_commands_parameter:
 
 Parameter Metadata