Question regarding __new__

[Frank Benkstein]

Hi,

the behaviour I always observed when creating instances by calling the
class A is that '__init__' is always only called when the object
returned by A.__new__ is an instance of A. This can be observed by the
following code:

class A(object):
def __new__(cls, *args, **kwds):
print "A.__new__", args, kwds
return object.__new__(B, *args, **kwds)
def __init__(cls, *args, **kwds):
print "A.__init__", args, kwds

class B(object):
def __new__(cls, *args, **kwds):
print "B.__new__", args, kwds
return object.__new__(cls, *args, **kwds)
def __init__(cls, *args, **kwds):
print "B.__init__", args, kwds

Interactively A() then gives:
A.__new__ () {}
<__main__.B object at 0xb7bed0ec>

Yet [1] says: "[...] some rules for __new__: [...] If you return an
object of a different class, its __init__ method will be called."

Am I missing something? Is this documented somewhere else? Also it
would be nice if someone could point me to the function that implements
this in C. I didn't find anything in object.c or typeobject.c.

Best regards
Frank Benkstein.

[1] http://www.python.org/download/releases/2.2/descrintro/#__new__

--
GPG (Mail): 7093 7A43 CC40 463A 5564 599B 88F6 D625 BE63 866F
GPG (XMPP): 2243 DBBA F234 7C5A 6D71 3983 9F28 4D03 7110 6D51

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.6 (GNU/Linux)

iD8DBQFGAqyuiPbWJb5jhm8RArUoAJwJJ0Dokhyx0wAWT9C/oXFIK5ua6ACfb3nA
pnZDNpMzkaHCJ2V3llTaPFo=
=yX4H
-----END PGP SIGNATURE-----

 

[Dustan]

Hi,

the behaviour I always observed when creating instances by calling the
class A is that '__init__' is always only called when the object
returned by A.__new__ is an instance of A. This can be observed by the
following code:

class A(object):
def __new__(cls, *args, **kwds):
print "A.__new__", args, kwds
return object.__new__(B, *args, **kwds)
def __init__(cls, *args, **kwds):
print "A.__init__", args, kwds

class B(object):
def __new__(cls, *args, **kwds):
print "B.__new__", args, kwds
return object.__new__(cls, *args, **kwds)
def __init__(cls, *args, **kwds):
print "B.__init__", args, kwds

Interactively A() then gives:

A.__new__ () {}
<__main__.B object at 0xb7bed0ec>

Yet [1] says: "[...] some rules for __new__: [...] If you return an
object of a different class, its __init__ method will be called."

Am I missing something? Is this documented somewhere else?
http://docs.python.org/ref/customization.html

Also it
would be nice if someone could point me to the function that implements
this in C. I didn't find anything in object.c or typeobject.c.

Best regards
Frank Benkstein.

[1]http://www.python.org/download/releases/2.2/descrintro/#__new__

--
GPG (Mail): 7093 7A43 CC40 463A 5564 599B 88F6 D625 BE63 866F
GPG (XMPP): 2243 DBBA F234 7C5A 6D71 3983 9F28 4D03 7110 6D51

signature.asc
1KDownload