旧式类 vs. 新式类（三）

2018-09-24 本文已影响0人 import_hello

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

8. 注脚

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