python __new__()和__init__()哪个更早？

通过代码验证是最靠谱的：

class Foo(object):    def __init__(self):        print 'foo init'    def __new__(cls,*args,**kwargs):        print 'foo new'        return object.__new__(cls,*args,**kwargs)        foo = Foo()print type(foo)

结果：

>>> foo newfoo init<class '__main__.Foo'>>>> 

可以看出来__new__()执行顺序比较早，实际上，新式类的__new__()才是真正的初始化函数。
Ps：cls表示一个类，一个当前要被实例化的类，参数由py解释器自动提供。


上述代码只能论证__new__比__init__更早被调用。但是why？

查了下官方文档：https://docs.python.org/2.7/reference/datamodel.html#object.__new__

object.__new__(cls[, ...])：
创建一个实例：Called to create a new instance of class cls.
静态方法：__new__() is a static method  that takes the class of which an instance was requested as its first argument. 
通过调用父辈的__new__：super(currentclass, cls).__new__(cls[, ...])
创建好实例才__init__：If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...])

object.__init__(cls[,...]):
有一句话：__new__() to create it, and __init__() to customise it

通过官方文档就能了解__new__()和__init__()的先后顺序了。但是why?

有的时候真讨厌自己喜欢寻根问底，好吧，直接上源码：C:\Python-2.7.9rc1\Objects\typeobject.c：

__init__()对应的实现代码：

static intobject_init(PyObject *self, PyObject *args, PyObject *kwds){    int err = 0;    if (excess_args(args, kwds)) {        PyTypeObject *type = Py_TYPE(self);        if (type->tp_init != object_init &&            type->tp_new != object_new)        {            err = PyErr_WarnEx(PyExc_DeprecationWarning,                       "object.__init__() takes no parameters",                       1);        }        else if (type->tp_init != object_init ||                 type->tp_new == object_new)        {            PyErr_SetString(PyExc_TypeError,                "object.__init__() takes no parameters");            err = -1;        }    }    return err;}

__new__()对应的实现代码：

static PyObject *object_new(PyTypeObject *type, PyObject *args, PyObject *kwds){    int err = 0;    if (excess_args(args, kwds)) {        if (type->tp_new != object_new &&            type->tp_init != object_init)        {            err = PyErr_WarnEx(PyExc_DeprecationWarning,                       "object() takes no parameters",                       1);        }        else if (type->tp_new != object_new ||                 type->tp_init == object_init)        {            PyErr_SetString(PyExc_TypeError,                "object() takes no parameters");            err = -1;        }    }    if (err < 0)        return NULL;    if (type->tp_flags & Py_TPFLAGS_IS_ABSTRACT) {        static PyObject *comma = NULL;        PyObject *abstract_methods = NULL;        PyObject *builtins;        PyObject *sorted;        PyObject *sorted_methods = NULL;        PyObject *joined = NULL;        const char *joined_str;        /* Compute ", ".join(sorted(type.__abstractmethods__))           into joined. */        abstract_methods = type_abstractmethods(type, NULL);        if (abstract_methods == NULL)            goto error;        builtins = PyEval_GetBuiltins();        if (builtins == NULL)            goto error;        sorted = PyDict_GetItemString(builtins, "sorted");        if (sorted == NULL)            goto error;        sorted_methods = PyObject_CallFunctionObjArgs(sorted,                                                      abstract_methods,                                                      NULL);        if (sorted_methods == NULL)            goto error;        if (comma == NULL) {            comma = PyString_InternFromString(", ");            if (comma == NULL)                goto error;        }        joined = PyObject_CallMethod(comma, "join",                                     "O",  sorted_methods);        if (joined == NULL)            goto error;        joined_str = PyString_AsString(joined);        if (joined_str == NULL)            goto error;        PyErr_Format(PyExc_TypeError,                     "Can't instantiate abstract class %s "                     "with abstract methods %s",                     type->tp_name,                     joined_str);    error:        Py_XDECREF(joined);        Py_XDECREF(sorted_methods);        Py_XDECREF(abstract_methods);        return NULL;    }    return type->tp_alloc(type, 0);}

我们就关注问题本身，why __new__比__init__更早，看到python源代码（C语言实现）：
可以看到object_init()实际上并没有什么代码，只是两个if判断，而object_new()才是各种属性搞起：

static PyObject *comma = NULL;
PyObject *abstract_methods = NULL;
PyObject *builtins;
PyObject *sorted;
PyObject *sorted_methods = NULL;
PyObject *joined = NULL;

所以问题得到解决：__new__()的确是创建一个新的实例，__init__()在实例上面进行customize（定制）。

貌似__new__()是新式类（继承自object）内置有的。