sorting question

[Ksenia Marasanova]

Hi,

I have a list that contains nodes from a tree. Each node is a class
instance, but I'll use dictionary here to simplify the example.
So the list looks like this:
[
{'id': 1,
'name': 'Parent node',
'ord_number': 1,
'parent_id': 0,
'url': '/parentnode/'},
{'id': 2,
'name': 'My node',
'ord_number': 1,
'parent_id': 1,
'url': '/parentnode/mynode/'}
]

Where 'ord_number' is the "sibling index". It's not always properly
filled in; sometimes it's the same for several siblings, or doesn't
starts with 1.

I want to sort this list with the following rules:
1. The parent must always come before the children in the list
2. Nodes with the same parent must be sorted by 'ord_number' 

The first rule is easy, cause I can use 'url' for it. List with nodes
is coming from the database, so I just do "ORDER BY url".
The second rule is kind of tricky to do in the database. I probably
would need to do something like "ORDER BY
cut_off_lastpart_from_url(url), ord_number". But there seems to be no
native string function in Postgres to do it easily, so I desided to
sort it in Python.

So I've come up with this:

        def cmp_tree(x, y):
            if x['parent_id'] == y['parent_id']:
                return cmp(x['ord_number'], y['ord_number'])
            else: 
                return cmp(x['url'], y['url'])

        nodes.sort(cmp_tree)

but it doesn't work as expected. Apparently I don't have a clue about
how sorting function work :(

Can anybody help?

-- 
Ksenia