Adds standardised attributes to BadArgument exceptions in discord.py

more_helpful_badargument_errors.py

"""Adds standardised attributes to BadArgument exceptions in discord.py.

The default `BadArgument` exception does not have any extra arguments,
and the error messages generated are too different to easily convert
into user-facing error messages.

This extension fixes that by capturing any `BadArgument` exceptions
raised and re-raising them with three extra arguments:
  
  * argument {str} -- The string that could not be converted
  * annotation {Any} -- The object the argument was annotated with
  * type_name {str} -- A formatted name of the target type

The `type_name` argument will generally return the name of the class.
If the class name ends in "Converter", this suffix is stripped off.

"""

import inspect
from typing import Any
from discord.ext import commands


async def _inject_argument_info(self, ctx: commands.Context,
                                converter: commands.Converter, argument: str,
                                param: inspect.Parameter) -> Any:
    """Add argument and converter info to `BadArgument` exceptions.

    This method is a replacement for the original `_actual_conversion`
    method of the `commands.Command` class. It forwards the call to the
    original method, but injects any `commands.BadArgument` exceptions
    raised with additional attributes to facilitate error message
    generation.

    """
    # NOTE: This method forwards the call to the original _actual_conversion
    # method. Whenever a BadArgument exception is raised, it is injected with
    # useful arguments before being re-raised.
    # This should be more-or-less invisible to any other modules, aside from
    # the additional call and the injected arguments.
    try:
        return await self._original_actual_conversion(ctx, converter,
                                                      argument, param)
    except commands.BadArgument as err:
        err.argument = argument
        err.annotation = param.annotation
        # Remove the "Converter"-suffix of object-specific converters. This
        # turns "MemberConverter" into "Member", etc.
        err.type_name = param.annotation.__name__.split('Converter')[:-1]
        raise err


def setup(bot: commands.Bot) -> None:
    """Entry point for the bot extension loader."""
    cmd_cls = commands.Command
    # Store the original converter
    cmd_cls._original_actual_conversion = cmd_cls._actual_conversion
    # Add the BadArgument attribute injector
    cmd_cls._actual_conversion = _inject_argument_info


def teardown(bot: commands.Bot) -> None:
    """Clean-up utility when unloading the extension."""
    cmd_cls = commands.Command
    # Restore the original converter
    cmd_cls._actual_conversion = cmd_cls._original_actual_conversion
    # Remove the backup
    del cmd_cls._original_actual_conversion