The difference between python new-style classes and old-style classes

Python’s new-style classes were introduced in version 2.2. We can call the previous classes classic classes or old-style classes.

Why is new style class introduced in 2.2? The official explanation is:

To unify classes and types.

Before 2.2, for example, in version 2.1, classes and types were different. If a is an instance of ClassA, then a.__class__ returns ‘class __main__.ClassA‘, and type(a) returns always . After introducing a new class, for example, ClassB is a new class, b is an instance of ClassB, b.__class__ and type(b) both return 'class '__main__.ClassB', so it is unified.

After introducing new classes, there are other benefits, such as more built-in properties will be introduced, descriptors can be introduced, properties can be calculated, etc.

For forward compatibility, user-defined classes are classic classes by default. New classes need to inherit from the base class object of all classes or a new class that inherits from object.

It is worth noting that although you are using the latest python (2.7), some features will not work on old-style classes.

So, in order to ensure that you are using new-style classes, there are two methods:

1. Metaclass, add the following code at the front of the class module code __metaclass__ = classname (a custom new-style class).

2. Classes inherit directly or indirectly from the built-in class object.

If you don’t need to be compatible with old-style classes and old versions of classes, then keep them all new-style classes.

In Python3, these problems no longer exist, because all classes are subclasses of the object class (implicitly).

Code example:

class oldClass:            #经典类
    def __init__( self ):
        pass
  
class newClass(object):    #新类
    def __init__( self ):
        pass 
  
c1 = oldClass()
c2 = newClass()
  
c1.__class__            # 输出-> <class __main__.oldClass at 0x0137BF10>
type(c1)                # 输出-> <type &#39;instance&#39;>
  
c2.__class__            # 输出-><class &#39;__main__.newClass&#39;>
type(c2)                # 输出-><class &#39;__main__.newClass&#39;>