def __new__(cls, *args, **kwargs):
print 'hi, i am Fo'
return super(Fo, cls).__new__(cls, *args, **kwargs)
class Foo(Fo):
__instance = None
def __new__(cls, *args, **kwargs):
if not cls.__instance:
print Foo is cls
print issubclass(cls, Fo)
print issubclass(cls, object)
cls.__instance = super(Foo, cls).__new__(cls, *args, **kwargs)
return cls.__instance
def hi(self):
print 'hi, world'
if __name__ == '__main__':
foo1 = Foo()
foo1.hi()
print isinstance(foo1, Foo)
print isinstance(foo1, Fo)
print isinstance(foo1, object)
运行结果如下：
True
True
True
hi, i am Fo
hi, world
True
True
True
如果如下定义Fo，也正常运行：
class Fo(object):
pass
但是，若这样定义：
class Fo(object):
def __new__(cls, *args, **kwargs):
print 'hi, i am Fo'
运行时报错如下：
AttributeError: 'NoneType' object has no attribute 'hi'
