Python does not execute __init__

Ask questions in doubt

The day before yesterday colleagues asked me a question, why the script does not call a __init__. The script is as Follows:

1 classA (object):2     def __init__(self, *args, * *kwargs):3         Print "call Init from%s"%self.__class__4 5     def __new__(cls, *args, * *kwargs):6obj = Object.__new__(cls, *args, * *Kwargs)7         Print "call New from%s"%obj.__class__8         returnobj9 Ten  one classB (object): a     def __init__(self, *args, * *kwargs): -         Print "call Init from%s"%self.__class__ -  the     def __new__(cls, *args, * *kwargs): -obj = Object.__new__(A, *args, * *Kwargs) -         Print "call New from%s"%obj.__class__ -         returnobj +  -b = b ()

In fact, I am also very strange, this script is very strange, the __new__ of Class B returns an instance of A. Only the __new__ method of B is executed, and the __init__ method of a is not executed.

Deep lost

Encounter this problem:

To go into the first look at the compiled Python instructions for the code, see b, the instructions for the __init__ method of b,

For example, to see how the B () method is called, a method test_b is Added. B () Show just two instructions

Load_global

Call_function

View the Python source code to

Pyobject_call (pyobject *func, pyobject *arg, pyobject *kw)
{
Ternaryfunc call;

If (call = func->ob_type->tp_call)! = NULL) {
Pyobject *result;
If (py_enterrecursivecall ("while calling a Python Object")
Return NULL;
result = (*call) (func, arg, kw);

See here when a little lost, do not know what Tp_call is a thing, not sure by the Typeobject to operate

(this must be reviewed because Tp_call has already made clear where he came From) ***************

Debugging Problem Solving

Finally resorted to, the big recruit to the Python source code to debug, compiles the Python source code on the Linux to add the parameter--with-debug, then executes gdb -ex r --args python test.py ,

After the Call_function,do_call,pyobject_call executes, make a Breakpoint. Saw him running to the

Type_call (pytypeobject *type, pyobject *args, pyobject *kwds)
{
obj = type->tp_new (type, args, kwds);
If (obj! = NULL) {
If (! Pytype_issubtype (obj->ob_type, Type))
Return obj;

, i'll see the code at this Point. Find out where the notes have been written:

/* If The returned object is not a instance of type,
It won ' t be initialized. */

well, It's clear. Does not produce a instance that is consistent with the class type, it is not initialized. That is called __init__.

The problem is Solved. Also learned the

Ask me how I know to break in that place, because I have read the code, just understand not so Deep.

Python does not execute __init__