REF: assume we get a decorated class in validate.

attr/_make.py

@@ -390,15 +390,9 @@ def validate(inst):
     if _config._run_validators is False:
         return
 
-    attributes = getattr(inst.__class__, "__attrs_attrs__", None)
-    if attributes is not None:
-        for a in attributes:
+    for a in inst.__class__.__attrs_attrs__:
         if a.validator is not None:
             a.validator(inst, a, getattr(inst, a.name))
-    else:
-        raise ValueError("{cl!r} is not an attrs-decorated class.".format(
-            cl=inst.__class__
-        ))
 
 
 def _attrs_to_script(attrs):