utility for showing a warning when a class is subclassed

scrapy/utils/deprecate.py

@@ -9,3 +9,17 @@ def attribute(obj, oldattr, newattr, version='0.12'):
     warnings.warn("%s.%s attribute is deprecated and will be no longer supported "
         "in Scrapy %s, use %s.%s attribute instead" % \
         (cname, oldattr, version, cname, newattr), ScrapyDeprecationWarning, stacklevel=3)
+
+
+def warn_when_subclassed(mro_len, message, category=ScrapyDeprecationWarning):
+    """
+    Return a metaclass that causes classes to
+    issue a warning when they are subclassed.
+    """
+    class Metaclass(type):
+        def __init__(cls, name, bases, clsdict):
+            if len(cls.mro()) > mro_len:
+                warnings.warn(message, category, stacklevel=2)
+            super(Metaclass, cls).__init__(name, bases, clsdict)
+    return Metaclass
+