Getting around immutable default arguments for recursion

MRAB google at
Wed Jan 14 23:26:06 CET 2009

dpapathanasiou wrote:
> 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=[]):

The usual solution is:

def get_prior_versions (item_id, priors=None):
     if priors is None:
         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)

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