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 new
foo 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 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;
}

__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。