python __new__()和__init__()哪个更早？

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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)

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

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?

static int
object_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;
}

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);
}

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

貌似__new__()是新式类（继承自object）内置有的。

python __new__()和__init__()哪个更早？

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原文地址：http://blog.csdn.net/emaste_r/article/details/47322827