[commands]Add typing.Literal converter

discord/ext/commands/core.py

@@ -489,31 +489,52 @@ class Command(_BaseCommand):
             raise BadArgument(f'Converting to "{name}" failed for parameter "{param.name}".') from exc
 
     async def do_conversion(self, ctx, converter, argument, param):
-        try:
-            origin = converter.__origin__
-        except AttributeError:
-            pass
-        else:
-            if origin is typing.Union:
-                errors = []
-                _NoneType = type(None)
-                for conv in converter.__args__:
-                    # if we got to this part in the code, then the previous conversions have failed
-                    # so we should just undo the view, return the default, and allow parsing to continue
-                    # with the other parameters
-                    if conv is _NoneType and param.kind != param.VAR_POSITIONAL:
-                        ctx.view.undo()
-                        return None if param.default is param.empty else param.default
-
+        origin = typing.get_origin(converter)
+
+        if origin is typing.Union:
+            errors = []
+            _NoneType = type(None)
+            for conv in typing.get_args(converter):
+                # if we got to this part in the code, then the previous conversions have failed
+                # so we should just undo the view, return the default, and allow parsing to continue
+                # with the other parameters
+                if conv is _NoneType and param.kind != param.VAR_POSITIONAL:
+                    ctx.view.undo()
+                    return None if param.default is param.empty else param.default
+
+                try:
+                    value = await self.do_conversion(ctx, conv, argument, param)
+                except CommandError as exc:
+                    errors.append(exc)
+                else:
+                    return value
+
+            # if we're here, then we failed all the converters
+            raise BadUnionArgument(param, typing.get_args(converter), errors)
+
+        if origin is typing.Literal:
+            errors = []
+            conversions = {}
+            literal_args = tuple(self._flattened_typing_literal_args(converter))
+            for literal in literal_args:
+                literal_type = type(literal)
+                try:
+                    value = conversions[literal_type]
+                except KeyError:
                     try:
-                        value = await self._actual_conversion(ctx, conv, argument, param)
+                        value = await self._actual_conversion(ctx, literal_type, argument, param)
                     except CommandError as exc:
                         errors.append(exc)
+                        conversions[literal_type] = object()
+                        continue
                     else:
-                        return value
+                        conversions[literal_type] = value
+
+                if value == literal:
+                    return value
 
-                # if we're  here, then we failed all the converters
-                raise BadUnionArgument(param, converter.__args__, errors)
+            # if we're here, then we failed to match all the literals
+            raise BadLiteralArgument(param, literal_args, errors)
 
         return await self._actual_conversion(ctx, converter, argument, param)
 
@@ -995,15 +1016,14 @@ class Command(_BaseCommand):
         return ''
 
     def _is_typing_optional(self, annotation):
-        try:
-            origin = annotation.__origin__
-        except AttributeError:
-            return False
+        return typing.get_origin(annotation) is typing.Union and typing.get_args(annotation)[-1] is type(None)
 
-        if origin is not typing.Union:
-            return False
-
-        return annotation.__args__[-1] is type(None)
+    def _flattened_typing_literal_args(self, annotation):
+        for literal in typing.get_args(annotation):
+            if typing.get_origin(literal) is typing.Literal:
+                yield from self._flattened_typing_literal_args(literal)
+            else:
+                yield literal
 
     @property
     def signature(self):
@@ -1011,7 +1031,6 @@ class Command(_BaseCommand):
         if self.usage is not None:
             return self.usage
 
-
         params = self.clean_params
         if not params:
             return ''
@@ -1019,6 +1038,22 @@ class Command(_BaseCommand):
         result = []
         for name, param in params.items():
             greedy = isinstance(param.annotation, converters._Greedy)
+            optional = False  # postpone evaluation of if it's an optional argument
+
+            # for typing.Literal[...], typing.Optional[typing.Literal[...]], and Greedy[typing.Literal[...]], the
+            # parameter signature is a literal list of it's values
+            annotation = param.annotation.converter if greedy else param.annotation
+            origin = typing.get_origin(annotation)
+            if not greedy and origin is typing.Union:
+                union_args = typing.get_args(annotation)
+                optional = union_args[-1] is type(None)
+                if optional:
+                    annotation = union_args[0]
+                    origin = typing.get_origin(annotation)
+
+            if origin is typing.Literal:
+                name = '|'.join(f'"{v}"' if isinstance(v, str) else str(v)
+                                for v in self._flattened_typing_literal_args(annotation))
 
             if param.default is not param.empty:
                 # We don't want None or '' to trigger the [name=value] case and instead it should
@@ -1038,7 +1073,7 @@ class Command(_BaseCommand):
                     result.append(f'[{name}...]')
             elif greedy:
                 result.append(f'[{name}]...')
-            elif self._is_typing_optional(param.annotation):
+            elif optional:
                 result.append(f'[{name}]')
             else:
                 result.append(f'<{name}>')

discord/ext/commands/errors.py

@@ -23,6 +23,7 @@ DEALINGS IN THE SOFTWARE.
 """
 
 from discord.errors import ClientException, DiscordException
+import typing
 
 
 __all__ = (
@@ -62,6 +63,7 @@ __all__ = (
     'NSFWChannelRequired',
     'ConversionError',
     'BadUnionArgument',
+    'BadLiteralArgument',
     'ArgumentParsingError',
     'UnexpectedQuoteError',
     'InvalidEndOfQuotedStringError',
@@ -644,6 +646,8 @@ class BadUnionArgument(UserInputError):
             try:
                 return x.__name__
             except AttributeError:
+                if typing.get_origin(x) is not None:
+                    return repr(x)
                 return x.__class__.__name__
 
         to_string = [_get_name(x) for x in converters]
@@ -654,6 +658,36 @@ class BadUnionArgument(UserInputError):
 
         super().__init__(f'Could not convert "{param.name}" into {fmt}.')
 
+class BadLiteralArgument(UserInputError):
+    """Exception raised when a :data:`typing.Literal` converter fails for all
+    its associated values.
+
+    This inherits from :exc:`UserInputError`
+
+    .. versionadded:: 2.0
+
+    Attributes
+    -----------
+    param: :class:`inspect.Parameter`
+        The parameter that failed being converted.
+    literals: Tuple[Any, ...]
+        A tuple of values compared against in conversion, in order of failure.
+    errors: List[:class:`CommandError`]
+        A list of errors that were caught from failing the conversion.
+    """
+    def __init__(self, param, literals, errors):
+        self.param = param
+        self.literals = literals
+        self.errors = errors
+
+        to_string = [repr(l) for l in literals]
+        if len(to_string) > 2:
+            fmt = '{}, or {}'.format(', '.join(to_string[:-1]), to_string[-1])
+        else:
+            fmt = ' or '.join(to_string)
+
+        super().__init__(f'Could not convert "{param.name}" into the literal {fmt}.')
+
 class ArgumentParsingError(UserInputError):
     """An exception raised when the parser fails to parse a user's input.
 

docs/ext/commands/commands.rst

@@ -502,6 +502,27 @@ resumes handling, which in this case would be to pass it into the ``liquid`` par
 
     This converter only works in regular positional parameters, not variable parameters or keyword-only parameters.
 
+typing.Literal
+^^^^^^^^^^^^^^^^
+
+A :data:`typing.Literal` is a special type hint that requires the passed parameter to be equal to one of the listed values
+after being converted to the same type. For example, given the following:
+
+.. code-block:: python3
+
+    from typing import Literal
+
+    @bot.command()
+    async def shop(ctx, buy_sell: Literal['buy', 'sell'], amount: Literal[1, 2], *, item: str):
+        await ctx.send(f'{buy_sell.capitalize()}ing {amount} {item}(s)!')
+
+
+The ``buy_sell`` parameter must be either the literal string ``"buy"`` or ``"sell"`` and ``amount`` must convert to the
+``int`` ``1`` or ``2``. If ``buy_sell`` or ``amount`` don't match any value, then a special error is raised,
+:exc:`~.ext.commands.BadLiteralArgument`. Any literal values can be mixed and matched within the same :data:`typing.Literal` converter.
+
+Note that ``typing.Literal[True]`` and ``typing.Literal[False]`` still follow the :class:`bool` converter rules.
+
 Greedy
 ^^^^^^^^