Unexpected behavior when initializing class

Arnaud Delobelle arnodel at googlemail.com
Wed Nov 28 18:13:25 CET 2007


On Nov 28, 3:04 pm, Mel <mwil... at the-wire.com> wrote:
> Paul Rudin wrote:
> > "alfred.fa... at gmail.com" <alfred.fa... at gmail.com> writes:
> > A common paradigm to get round this - assuming you want a different
> > empty list each time - is something like:
>
> > def __init__(self, v = None):
> >     self.values = v if v else []
>
> > (or maybe test explicitly for None, but you get the idea.)
>
> Do test explicitly for None.  Otherwise, if you do
>
> a = []
> x = ThatClass (a)
>
> it will so happen that x.values will be an empty list, but it won't be
> the same list as a.
>
>         Mel.

Yes.  Another much safer possibility is to make a copy of the initial
v:

def __init__(self, values=[]):
    self.values = list(values)

As a nice side effect, the object can be initialised with any
iterable.

--
Arnaud




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