[commands] Ensure handlers are copied even during update.

Fix #2001

discord/ext/commands/core.py

@@ -262,26 +262,29 @@ class Command(_BaseCommand):
         """
         self.__init__(self.callback, **dict(self.__original_kwargs__, **kwargs))
 
-    def copy(self):
-        """Creates a copy of this :class:`Command`."""
-        ret = self.__class__(self.callback, **self.__original_kwargs__)
-        ret._before_invoke = self._before_invoke
-        ret._after_invoke = self._after_invoke
-        if self.checks != ret.checks:
-            ret.checks = self.checks.copy()
-        if self._buckets != ret._buckets:
-            ret._buckets = self._buckets.copy()
+    def _ensure_assignment_on_copy(self, other):
+        other._before_invoke = self._before_invoke
+        other._after_invoke = self._after_invoke
+        if self.checks != other.checks:
+            other.checks = self.checks.copy()
+        if self._buckets != other._buckets:
+            other._buckets = self._buckets.copy()
         try:
-            ret.on_error = self.on_error
+            other.on_error = self.on_error
         except AttributeError:
             pass
-        return ret
+        return other
+
+    def copy(self):
+        """Creates a copy of this :class:`Command`."""
+        ret = self.__class__(self.callback, **self.__original_kwargs__)
+        return self._ensure_assignment_on_copy(ret)
 
     def _update_copy(self, kwargs):
         if kwargs:
             copy = self.__class__(self.callback, **kwargs)
             copy.update(**self.__original_kwargs__)
-            return copy
+            return self._ensure_assignment_on_copy(copy)
         else:
             return self.copy()