[commands] add default __repr__ for CustomDefault

With this change CustomDefaults display nicer in help commands and
in command.signature

discord/ext/commands/default.py

@@ -34,7 +34,19 @@
     'Call',
 )
 
-class CustomDefault:
+class CustomDefaultMeta(type):
+    def __new__(cls, *args, **kwargs):
+        name, bases, attrs = args
+        attrs['display'] = kwargs.pop('display', name)
+        return super().__new__(cls, name, bases, attrs, **kwargs)
+
+    def __repr__(cls):
+        return str(cls)
+
+    def __str__(cls):
+        return cls.display
+
+class CustomDefault(metaclass=CustomDefaultMeta):
     """The base class of custom defaults that require the :class:`.Context`.
 
     Classes that derive from this should override the :meth:`~.CustomDefault.default`

docs/ext/commands/commands.rst

@@ -640,6 +640,10 @@ A DefaultParam returning ``None`` is valid - if this should be an error, raise :
                 my_bytes = io.BytesIO(await resp.content.read())
         await ctx.send(file=discord.File(filename="your_image", fp=my_bytes))
 
+.. tip:: You can change the name of a Custom Default that is displayed in help command by passing ``display`` meta option. Using previous example: ``class LastImage(CustomDefault, display='last image from chat')``
+
+
+
 
 Checks
 -------