Mutable default values for function parameters

[Brian Quinlan]

> 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 []