why doesn't this django mptt algorithm correctly compute rank? list index out

Question

I am trying to write a simple algorithm to get the 'rank' of a django mptt model. By 'rank' I mean the number of levels of descendents... similar to MPTTModel.get_level() but counting from leaf to root.

My approach is the following:

views.py

 def index(request):
    nodes = Node.objects.all()
    nodesets = []
    for i in nodes:
        figure_rank(i, i)
        if i.count >= 3:
            nodeset = Node_set(node=i, name=i.name)
            nodeset.save()      # This is where the error is caught
            nodesets.append(dataset)
    return render(request, 'associate/index.html', {'nodesets':nodesets})

def figure_rank(over, under):

    if under.get_children()[0]:
        r = under.get_children()[0]
        over.count += 1
        figure_rank(over, r)
    else:
        return over.count

For nodes with rank >=3 I want to create a Node_set and return a set of nodes once they are all given ranks.

I get the following error:

Environment:


Request Method: GET
Request URL: http://127.0.0.1:8000/associate/

Django Version: 1.6.2
Python Version: 2.7.5
Installed Applications:
('django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'associate',
 'images_app',
 'mptt')
Installed Middleware:
('django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware')


Traceback:
File "C:\Python27\lib\site-packages\django\core\handlers\base.py" in get_response
  114.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "C:\djangoprojects\images\associate\views.py" in index
  9.         figure_rank(i, i)
File "C:\djangoprojects\images\associate\views.py" in figure_rank
  23.         figure_rank(over, r)
File "C:\djangoprojects\images\associate\views.py" in figure_rank
  23.         figure_rank(over, r)
File "C:\djangoprojects\images\associate\views.py" in figure_rank
  20.     if under.get_children()[0]:
File "C:\Python27\lib\site-packages\django\db\models\query.py" in __getitem__
  132.         return list(qs)[0]

Exception Type: IndexError at /associate/
Exception Value: list index out of range

What am I doing wrong here?

Answer 1

You need to properly check if there are no children. In figure_rank, under.get_children() is returning an empty list, and then you try to access the first element. Check if the list is empty first.

def figure_rank(over, under):
    children = under.get_children()
    if chilren: # check if children is nonempty before accessing the first element
        r = children[0]
        over.count += 1
        figure_rank(over, r)
    else:
        return over.count