Le Sat, 21 Feb 2009 11:38:49 -0800,
Moos Heintzen <iwasr...@gmail.com> s'exprima ainsi:
> Hi,
>
> This behavior was totally unexpected. I only caught it because it was
> the only thing I changed.
>
> >>> class foo:
> ... def __init__(self, lst=[]):
> ... self.items = lst
> ...
> >>> f1 = foo()
> >>> f1.items
> []
> >>> f1.items.append(1)
> >>> f2 = foo()
> >>> f2.items
> [1]
>
> Huh? lst is a reference to the *same list* every instance?
Yop! Default args are evaluated once and only once at func def time. Very
common trap, indeed! Note that this has nothing to do with __init__, nore with
methods specifically. You can reproduce your example with a "free" function.
> I guess I have to do it like this. It seems to work. (i.e. every foo
> instance with default lst now has a unique new list.)
>
> def__init__(self, lst=None):
> self.items = lst or []
This is the right remedy. Except that I would write
self.items = [] if lst is None else lst
to avoid "tricking" with bools (personal taste).
denis
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la vita e estrany
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