strange behaviour when inheriting from tuple

ryles rylesny at gmail.com
Sun Oct 11 11:30:43 CEST 2009


On Oct 11, 3:04 am, metal <metal... at gmail.com> wrote:
> Environment:
>
> PythonWin 2.5.4 (r254:67916, Apr 27 2009, 15:41:14) [MSC v.1310 32 bit
> (Intel)] on win32.
> Portions Copyright 1994-2008 Mark Hammond - see 'Help/About PythonWin'
> for further copyright information.
>
> Evil Code:
>
> class Foo:
>         def __init__(self, *args):
>                 print args
>
> Foo(1, 2, 3) # (1, 2, 3), good
>
> class Bar(tuple):
>         def __init__(self, *args):
>                 print args
>
> Bar(1, 2, 3) # TypeError: tuple() takes at most 1 argument (3 given)
>
> what the heck? I even didn't call tuple.__init__ yet

When subclassing immutable types you'll want to override __new__, and
should ensure that the base type's __new__ is called:

__ class MyTuple(tuple):
__     def __new__(cls, *args):
__         return tuple.__new__(cls, args)
__ print MyTuple(1, 2, 3)

(1, 2, 3)

See http://www.python.org/download/releases/2.2.3/descrintro/#__new__



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