search results from a Django model method including the results from earlier calls to the same method (x post StackOverflow)

[yolabingo]

I also posted this to SO 
https://stackoverflow.com/questions/58600159/search-results-from-a-django-model-method-including-the-results-from-earlier-cal
hope that's ok, let me know if not.

I have a Django Model that tracks email message IDs as the message passes through different servers. It has a models.ForeignKey field to itself, so we can chain them together to follow the full path of a message through many servers. 

The model has a 'get_children()' method that recursively looks for all messages descended from the given message.

    class Relay(models.Model):

        hostname = models.CharField(max_length=48)

        qid = models.CharField(max_length=24)

        received_from = models.ForeignKey(

            "self",

            blank=True,

            null=True,

            default=None,

            on_delete=models.SET_DEFAULT

        )

        def get_children(self, parent_messages=set()):

            for r in Relay.objects.filter(received_from=self):

                r.get_children(parent_messages)

            parent_messages.add(self)

            p = list(parent_messages)

            parent_messages = set()

            return p

       

If I run a single query from the Django shell, it works as expected. "Solo" messages return only themselves. Messages with multiple child/descendant messages are found as well.

    >>> r = Relay.objects.get(qid='xo2')

    >>> r.get_children()

    [<Relay: server2 foo>, <Relay: server3 bbb>, <Relay: server1 xo2>]

If I kill and restart the shell, the next query works as expected, fetching a single message

    >>> r = Relay.objects.get(qid='singletonMsg')

    >>> r.get_children()

    [<Relay: server5 singletonMsg>]

But if I run get_children() repeatedly within a single Django shell session, it always includes the results of the previous get_children() calls.

   

    >>> r = Relay.objects.get(qid='singletonMsg')

    >>> r.get_children()

    # expected result

    [<Relay: server5 singletonMsg>]  

    >>>

    >>> r = Relay.objects.get(qid='xo2')

    >>> r.get_children()

    # unexpected result - should not include singletonMsg 

    [<Relay: server2 foo>, <Relay: server3 bbb>, <Relay: server5 singletonMsg>, <Relay: server1 xo2>]

    >>>

    >>> r = Relay.objects.get(qid='singletonMsg')

    >>> r.get_children()

    # unexected result - should just return singletonMsg ??

    [<Relay: server2 foo>, <Relay: server3 bbb>, <Relay: server5 singletonMsg>, <Relay: server1 xo2>]

I was originally returning the "parent_messages" set from the function. I tried 

    return [m for m in parent_messages]

and the current list() thinking it was a closure issue, but no luck.

I am stumped. Thanks in advance for any advice.

[yolabingo]

Solved on SO, but an extremely educational response.

I learned about mutable default argument gotcha https://stackoverflow.com/questions/1132941/least-astonishment-and-the-mutable-default-argument

And Django Modified Preorder Tree Traversal 
https://github.com/django-mptt/django-mptt