Getting around immutable default arguments for recursion

[dpapathanasiou]

I wrote this function to retrieve a list of items from a dictionary.

The first time it was called, it worked properly.

But every subsequent call returned the results of the prior call, plus
the results of the current call.

I was confused until I read in the docs that default arguments are
immutable.

Is there any way around this, to be able to write recursive functions
with default arguments?

Here's the code:

def get_prior_versions (item_id, priors=[]):
    """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:
        priors.append(prior_id)
        return get_prior_versions(prior_id, priors)