[commands] Fix displayed_default for callables and None values

discord/ext/commands/core.py

@@ -1209,7 +1209,7 @@ class Command(_BaseCommand, Generic[CogT, P, T]):
             if not param.required:
                 # We don't want None or '' to trigger the [name=value] case and instead it should
                 # do [name] since [name=None] or [name=] are not exactly useful for the user.
-                if param.default is not None and param.displayed_default:
+                if param.displayed_default:
                     result.append(
                         f'[{name}={param.displayed_default}]' if not greedy else f'[{name}={param.displayed_default}]...'
                     )

discord/ext/commands/parameters.py

@@ -175,7 +175,13 @@ class Parameter(inspect.Parameter):
         if self._displayed_default is not empty:
             return self._displayed_default
 
-        return None if self.required else str(self.default)
+        if self.required:
+            return None
+
+        if callable(self.default) or self.default is None:
+            return None
+
+        return str(self.default)
 
     @property
     def displayed_name(self) -> Optional[str]: