Initializing subclasses of tuple

[Dave Opstad]

I'm hoping someone can point out where I'm going wrong here. Here's a
snippet of a Python interactive session (2.3, if it makes a difference):

--------------------------------------

class X(list): .... def __init__(self, n):
.... v = range(n)
.... list.__init__(self, v)
.... x = X(10)
x [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] class Y(tuple): .... def __init__(self, n):
.... v = tuple(range(n))
.... tuple.__init__(self, v)
.... y = Y(10)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: iteration over non-sequence
--------------------------------------

How do I initialize instances of a class derived from tuple, if it's not
in the __init__ method?

Thanks for any help!
Dave Opstad

[Steve Holden]

Dave Opstad wrote:

I'm hoping someone can point out where I'm going wrong here. Here's a
snippet of a Python interactive session (2.3, if it makes a difference):

--------------------------------------

class X(list):
... def __init__(self, n):
... v = range(n)
... list.__init__(self, v)
...
x = X(10)
x
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
class Y(tuple):
... def __init__(self, n):
... v = tuple(range(n))
... tuple.__init__(self, v)
...
y = Y(10)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: iteration over non-sequence
--------------------------------------

How do I initialize instances of a class derived from tuple, if it's not
in the __init__ method?

In the __new__ method! This must return the actual created object,
whereas __init__ initializes the already-created object.

This applies to subclassing all the built-in types.

regards
Steve
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/

[Just]

In article <da*******************************@reader0903.news .uu.net>,
Dave Opstad <da*********@monotypeimaging.com> wrote:

I'm hoping someone can point out where I'm going wrong here. Here's a
snippet of a Python interactive session (2.3, if it makes a difference):

--------------------------------------

class X(list): ... def __init__(self, n):
... v = range(n)
... list.__init__(self, v)
... x = X(10)
x [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] class Y(tuple): ... def __init__(self, n):
... v = tuple(range(n))
... tuple.__init__(self, v)
... y = Y(10)

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: iteration over non-sequence
--------------------------------------

How do I initialize instances of a class derived from tuple, if it's not
in the __init__ method?

Hi Dave,

You're going to have to override __new__. See eg.
http://groups-beta.google.com/group/..._thread/thread
/4a53d2c69209ba76/9b21a8326d0ef002

http://mail.python.org/pipermail/tut...ry/027779.html

Good luck,

Just

[gry]

To inherit from an immutable class, like string or tuple, you need to
use the __new__ member, not __init__. See, e.g.:

http://www.python.org/2.2.3/descrintro.html#__new__

[Nick Coghlan]

gr*@ll.mit.edu wrote:

To inherit from an immutable class, like string or tuple, you need to
use the __new__ member, not __init__. See, e.g.:

http://www.python.org/2.2.3/descrintro.html#__new__

Any volunteers out there willing to put some words about __new__ together for
the main Python docs, please consider posting them on SF :)

It's currently conspicuous by its absence:
http://www.python.org/dev/doc/devel/...omization.html

Cheers,
Nick.

--
Nick Coghlan | nc******@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net

[Steven Bethard]

Nick Coghlan wrote:

Any volunteers out there willing to put some words about __new__
together for the main Python docs, please consider posting them on SF :)

Done.

http://sourceforge.net/tracker/?func...70&atid=105470

STeVe