Mutable default values for function parameters

[Steve Holden]

"Brian Quinlan" <BrianQ at ActiveState.com> wrote in message
news:mailman.1002222005.17547.python-list at python.org...
> > I've used this idiom myself many times. I eventuallu decided
> > it was shorter
> > as
> >
> > def sample(x, d = None):
> >     d = d or []
> >     d.append(x)
> >     print d
> >
> > but this looks so weird I'd appreciate confirmation that it's a
> valid
> > replacement.
>
> It is not a valid replacement because d can evaluate to zero for
> values other than None e.g.
>
> sample(x, '')
> sample(x, 0)
>
> Those should raise some sort of exception, not silently do the
> default.
>
> You could change it to:
> d = (d is None) or []

Tanks, I remember finding the particular fly you metnion in the ointment. I
could conveniently ignore it for my needs, however, as all non-default
values were also non-false. However, your solution won't work at all,
because (d is None) evaluates to 1, which would be used instead of the
default value! Same goes for (d == None) I'm afraid.

Things are never as easy as they seem.

regards
 Steve
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