旧式类 vs. 新式类（三）

转载须注明出处：简书@Orca_J35

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4. 重要差异

引入新式类的主要动机是为了提供具有完整元模型(meta-model)的统一对象模型。新式类还具备许多实用功能：可将大多数内置类型子类化、引入了"描述符(descriptors)"等。

新式类和旧式类除了 type() 的返回值不同以外，两者在许多重要的细节上也拥有完全不同的行为。"新对象模型"与"旧对象模型"相比，一些行为拥有根本性的差异，比如调用特殊方法的方式。另外，"新对象模型"还对之前的部分行为进行"修正(fixes)"，比如在多重继承中的方法解析顺序。

新式类较旧式类的重要差异如下：^{[1] [2]}

• 内置函数 super 仅支持新式类

• 新式类支持描述符(descriptors)，可阅读 【译】Python描述符指南

• 新式类支持装饰器(decorators) - introduced in Python 2.4

• 新式类支持静态方法和类方法

• 新式类增加了 __new__ 方法

• 新式类增加了 __getattribute__() 方法

• 新式类增加了 __slots__ 方法

• properties (computed attributes)

• MRO 的算法更新，可阅读 Method Resolution Order - Python 见闻志

  Classic classes do a depth first search from left to right. Stop on first match. They do not have the __mro__ attribute.

  class C: i = 0
  class C1(C): pass
  class C2(C): i = 2
  class C12(C1, C2): pass
  class C21(C2, C1): pass
  
  assert C12().i == 0
  assert C21().i == 2
  
  try:
      C12.__mro__
  except AttributeError:
      pass
  else:
      assert False
  

  New-style classes

  class C(object): i = 0
  class C1(C): pass
  class C2(C): i = 2
  class C12(C1, C2): pass
  class C21(C2, C1): pass
  
  assert C12().i == 2
  assert C21().i == 2
  
  assert C12.__mro__ == (C12, C1, C2, C, object)
  assert C21.__mro__ == (C21, C2, C1, C, object)
  

• 在抛出异常时，可使用任意旧式类，但只能使用继承自 Exception 的新式类。

  # in Python 2.7
  # OK, old:
  class Old: pass
  try:
      raise Old()
  except Old:
      pass
  else:
      assert False
  
  # TypeError, new not derived from `Exception`.
  class New(object): pass
  try:
      raise New()
  except TypeError:
      pass
  else:
      assert False
  
  # OK, derived from `Exception`.
  class New(Exception): pass
  try:
      raise New()
  except New:
      pass
  else:
      assert False
  
  # `'str'` is a new style object, so you can't raise it:
  try:
      raise 'str'
  except TypeError:
      pass
  else:
      assert False
  

6. 术语

classic class

in Glossary of Python 2

Any class which does not inherit from object. See new-style class. Classic classes have been removed in Python 3.

new-style class

Any class which inherits from object. This includes all built-in types like list and dict. Only new-style classes can use Python's newer, versatile features like __slots__, descriptors, properties, and __getattribute__(), class methods, and static methods.

More information can be found in New-style and classic classes.

7. 扩展阅读

• https://wiki.python.org/moin/NewClassVsClassicClass

• https://www.python.org/doc/newstyle/

8. 注脚

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1. What is the difference between old style and new style classes in Python? ↩

2. The Inside Story on New-Style Classes ↩