1 # Copyright 2017 The Chromium Authors. All rights reserved. 2 # Use of this source code is governed by a BSD-style license that can be 3 # found in the LICENSE file. 4 5 class SlotsMetaclass(type): 6 """This metaclass requires all subclasses to define __slots__. 7 8 Usage: 9 class Foo(object): 10 __metaclass__ = slots_metaclass.SlotsMetaclass 11 __slots__ = '_property0', '_property1', 12 13 __slots__ must be a tuple containing string names of all properties that the 14 class contains. 15 Defining __slots__ reduces memory usage, accelerates property access, and 16 prevents dynamically adding unlisted properties. 17 If you need to dynamically add unlisted properties to a class with this 18 metaclass, then take a step back and rethink your goals. If you really really 19 need to dynamically add unlisted properties to a class with this metaclass, 20 add '__dict__' to its __slots__. 21 """ 22 23 def __new__(mcs, name, bases, attrs): 24 assert '__slots__' in attrs, 'Class "%s" must define __slots__' % name 25 assert isinstance(attrs['__slots__'], tuple), '__slots__ must be a tuple' 26 27 return super(SlotsMetaclass, mcs).__new__(mcs, name, bases, attrs) 28
