renumbering chapters >= 19

24-class-metaprog/slots/slots_timing.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+
+class Wrong:
+
+    def __init_subclass__(subclass):
+        subclass.__slots__ = ('x', 'y')
+
+
+class Klass0(Wrong):
+    pass
+
+
+o = Klass0()
+o.z = 3
+print('o.z = 3  # did not raise Attribute error because __slots__ was created too late')
+
+
+class Correct1(type):
+
+    def __new__(meta_cls, cls_name, bases, cls_dict):
+        cls_dict['__slots__'] = ('x', 'y')
+        return super().__new__(
+                    meta_cls, cls_name, bases, cls_dict)
+
+
+class Klass1(metaclass=Correct1):
+    pass
+
+o = Klass1()
+try:
+    o.z = 3
+except AttributeError as e:
+    print('Raised as expected:', e)
+
+
+class Correct2(type):
+    def __prepare__(name, bases):
+        return dict(__slots__=('x', 'y'))
+
+class Klass2(metaclass=Correct2):
+    pass
+
+o = Klass2()
+try:
+    o.z = 3
+except AttributeError as e:
+    print('Raised as expected:', e)
+