从新型和经典类:

Up to Python 2.1, old-style classes were the only flavour available to the user. The concept of (old-style) class is unrelated to the concept of type: if x is an instance of an old-style class, then x.__class__ designates the class of x, but type(x) is always <type 'instance'>. This reflects the fact that all old-style instances, independently of their class, are implemented with a single built-in type, called instance. New-style classes were introduced in Python 2.2 to unify the concepts of class and type. A new-style class is simply a user-defined type, no more, no less. If x is an instance of a new-style class, then type(x) is typically the same as x.__class__ (although this is not guaranteed – a new-style class instance is permitted to override the value returned for x.__class__). The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model. It also has a number of immediate benefits, like the ability to subclass most built-in types, or the introduction of "descriptors", which enable computed properties. For compatibility reasons, classes are still old-style by default. New-style classes are created by specifying another new-style class (i.e. a type) as a parent class, or the "top-level type" object if no other parent is needed. The behaviour of new-style classes differs from that of old-style classes in a number of important details in addition to what type returns. Some of these changes are fundamental to the new object model, like the way special methods are invoked. Others are "fixes" that could not be implemented before for compatibility concerns, like the method resolution order in case of multiple inheritance. Python 3 only has new-style classes. No matter if you subclass from object or not, classes are new-style in Python 3.

New-style类继承自object，并且在Python 2.2以后必须这样编写(即类Classname(object):而不是类Classname:)。核心变化是统一类型和类，这样做的好处是允许从内置类型继承。

更多细节请阅读描述介绍。

Declaration-wise:

New-style类继承自object或另一个New-style类。

class NewStyleClass(object):
    pass

class AnotherNewStyleClass(NewStyleClass):
    pass

老式的类没有。

class OldStyleClass():
    pass

注意事项:

Python 3不支持旧样式的类，所以上面提到的任何一种形式都会产生新样式的类。

对于属性查找，旧样式的类仍然稍微快一些。这通常并不重要，但在性能敏感的Python 2中可能很有用。x代码:

In [3]: class A:
   ...:     def __init__(self):
   ...:         self.a = 'hi there'
   ...:

In [4]: class B(object):
   ...:     def __init__(self):
   ...:         self.a = 'hi there'
   ...:

In [6]: aobj = A()
In [7]: bobj = B()

In [8]: %timeit aobj.a
10000000 loops, best of 3: 78.7 ns per loop

In [10]: %timeit bobj.a
10000000 loops, best of 3: 86.9 ns per loop

Guido写了关于新型类的内幕，这是一篇关于Python中的新型和老式类的很棒的文章。

Python 3只有new-style类。即使你写了一个“老式类”，它也是隐式地从object派生的。

新型类具有一些老式类所缺乏的高级特性，如super，新的C3 mro，一些神奇的方法等。

新的样式类可以使用super(Foo, self)，其中Foo是类，self是实例。

超级(类型、对象或者类型) 返回一个代理对象，该对象将方法调用委托给类型的父类或兄弟类。这对于访问在类中被重写的继承方法非常有用。除了类型本身被跳过之外，搜索顺序与getattr()所使用的相同。

在Python 3中。X你可以简单地在类中使用super()而不带任何参数。

这里有一个非常实际的真/假的区别。以下代码的两个版本之间的唯一区别是，在第二个版本中，Person从object继承。除此之外，这两个版本是相同的，但结果不同:

Old-style classes class Person(): _names_cache = {} def __init__(self,name): self.name = name def __new__(cls,name): return cls._names_cache.setdefault(name,object.__new__(cls,name)) ahmed1 = Person("Ahmed") ahmed2 = Person("Ahmed") print ahmed1 is ahmed2 print ahmed1 print ahmed2 >>> False <__main__.Person instance at 0xb74acf8c> <__main__.Person instance at 0xb74ac6cc> >>> New-style classes class Person(object): _names_cache = {} def __init__(self,name): self.name = name def __new__(cls,name): return cls._names_cache.setdefault(name,object.__new__(cls,name)) ahmed1 = Person("Ahmed") ahmed2 = Person("Ahmed") print ahmed2 is ahmed1 print ahmed1 print ahmed2 >>> True <__main__.Person object at 0xb74ac66c> <__main__.Person object at 0xb74ac66c> >>>

新旧样式类之间的重要行为变化

超级补充道 MRO变更(解释如下) 描述符添加 除非派生自Exception，否则不能引发new样式类对象(如下例) __slots__补充道

MRO(方法解决顺序)更改

它在其他回答中提到过，但这里有一个经典MRO和C3 MRO之间区别的具体例子(用于新风格的职业)。

问题是在多重继承中搜索属性(包括方法和成员变量)的顺序。

经典类从左到右进行深度优先搜索。停在第一根火柴上。它们没有__mro__属性。

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

新型课程MRO在一个英语句子中合成起来更加复杂。这里有详细的解释。它的一个属性是只搜索一次基类的所有派生类。它们有__mro__属性，显示搜索顺序。

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)

除非派生自Exception，否则不能引发新的样式类对象

在Python 2.5左右，可以引发许多类，在Python 2.6左右，这被删除了。Python 2.7.3:

# 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