[Tutor] Default list arguments in __init__

[Kent Johnson]

On Sat, Feb 21, 2009 at 2:55 PM, spir <denis.spir at free.fr> wrote:
> Le Sat, 21 Feb 2009 11:38:49 -0800,
> Moos Heintzen <iwasroot at gmail.com> s'exprima ainsi:
>
>> Hi,
>>
>> This behavior was totally unexpected. I only caught it because it was
>> the only thing I changed.

Yes, it is a common trap and a FAQ:
http://effbot.org/pyfaq/why-are-default-values-shared-between-objects.htm

>> 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).

These two versions have different behaviour if an empty list is passed
in. Most likely the second one will be what was intended.

Kent