Question

I’m trying to work out if my problem is solvable using the builtin sorted() function or if I need to do myself – old school using cmp would have been relatively easy.

My data-set looks like:

x = [
('business', Set('fleet','address'))
('device', Set('business','model','status','pack'))
('txn', Set('device','business','operator'))
....

The sort rule should be basically for all value of N & Y where Y > N, x[N][0] not in x[Y][1]

Although I’m using Python 2.6 where the cmp argument is still available I’m trying to make this Python 3 safe.

So, can this be done using some lambda magic and the key argument?

-== UPDATE ==-

Thanks Eli & Winston! I didn’t really think using key would work, or if it could I suspected it would be a shoe horn solution which isn’t ideal.

Because my problem was for database table dependencies I had to make a minor addition to Eli’s code to remove an item from its list of dependencies (in a well designed database this wouldn’t happen, but who lives in that magical perfect world?)

My Solution:

def topological_sort(source):
    """perform topo sort on elements.

    :arg source: list of ``(name, set(names of dependancies))`` pairs
    :returns: list of names, with dependancies listed first
    """
    pending = [(name, set(deps)) for name, deps in source]        
    emitted = []
    while pending:
        next_pending = []
        next_emitted = []
        for entry in pending:
            name, deps = entry
            deps.difference_update(set((name,)), emitted) # <-- pop self from dep, req Py2.6
            if deps:
                next_pending.append(entry)
            else:
                yield name
                emitted.append(name) # <-- not required, but preserves original order
                next_emitted.append(name)
        if not next_emitted:
            raise ValueError("cyclic dependancy detected: %s %r" % (name, (next_pending,)))
        pending = next_pending
        emitted = next_emitted

Answer 1

What you want is called a topological sort. While it’s possible to implement using the builtin sort(), it’s rather awkward, and it’s better to implement a topological sort directly in python.

Why is it going to be awkward? If you study the two algorithms on the wiki page, they both rely on a running set of “marked nodes”, a concept that’s hard to contort into a form sort() can use, since key=xxx (or even cmp=xxx) works best with stateless comparison functions, particularly because timsort doesn’t guarantee the order the elements will be examined in. I’m (pretty) sure any solution which does use sort() is going to end up redundantly calculating some information for each call to the key/cmp function, in order to get around the statelessness issue.

The following is the alg I’ve been using (to sort some javascript library dependancies):

edit: reworked this greatly, based on Winston Ewert’s solution

def topological_sort(source):
    """perform topo sort on elements.

    :arg source: list of ``(name, [list of dependancies])`` pairs
    :returns: list of names, with dependancies listed first
    """
    pending = [(name, set(deps)) for name, deps in source] # copy deps so we can modify set in-place       
    emitted = []        
    while pending:
        next_pending = []
        next_emitted = []
        for entry in pending:
            name, deps = entry
            deps.difference_update(emitted) # remove deps we emitted last pass
            if deps: # still has deps? recheck during next pass
                next_pending.append(entry) 
            else: # no more deps? time to emit
                yield name 
                emitted.append(name) # <-- not required, but helps preserve original ordering
                next_emitted.append(name) # remember what we emitted for difference_update() in next pass
        if not next_emitted: # all entries have unmet deps, one of two things is wrong...
            raise ValueError("cyclic or missing dependancy detected: %r" % (next_pending,))
        pending = next_pending
        emitted = next_emitted

---

Sidenote: it is possible to shoe-horn a cmp() function into key=xxx, as outlined in this python bug tracker message.