Getting around immutable default arguments for recursion

[dpapathanasiou]

> How about:
>
> def get_prior_versions (item_id, priors=None):
>    """Return a list of all prior item ids starting with this one"""
>    global history_db # key = item id, value = prior item id
>    prior_id = history_db[item_id]
>    if not prior_id:
>        return priors
>    else:
>        if priors:
>            priors.append(prior_id)
>        else:
>            priors = [prior_id]
>        return get_prior_versions(prior_id, priors)

It's not exactly right for what I'm doing, b/c the caller always
expects a list in return.