python __new__()和__init__()哪個更早？

標籤：

通過代碼驗證是最靠譜的：

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)

結果：

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

可以看出來__new__()執行順序比較早，實際上，新式類的__new__()才是真正的初始化函數。
Ps：cls表示一個類，一個當前要被執行個體化的類，參數由py解譯器自動提供。


上述代碼只能論證__new__比__init__更早被調用。但是why？

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

object.__new__(cls[, ...])：
建立一個執行個體：Called to create a new instance of class cls.
靜態方法：__new__() is a static method  that takes the class of which an instance was requested as its first argument. 
通過調用父輩的__new__：super(currentclass, cls).__new__(cls[, ...])
建立好執行個體才__init__：If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...])

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

通過官方文檔就能瞭解__new__()和__init__()的先後順序了。但是why?

有的時候真討厭自己喜歡尋根問底，好吧，直接上源碼：C:\Python-2.7.9rc1\Objects\typeobject.c：

__init__()對應的實現代碼：

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__()對應的實現代碼：

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);}

我們就關注問題本身，why __new__比__init__更早，看到python原始碼（C語言實現）：
可以看到object_init()實際上並沒有什麼代碼，只是兩個if判斷，而object_new()才是各種屬性搞起：

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

所以問題得到解決：__new__()的確是建立一個新的執行個體，__init__()在執行個體上面進行customize（定製）。

貌似__new__()是新式類（繼承自object）內建有的。

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python __new__()和__init__()哪個更早？