Recursively creating a tree hierarchy without using class/object

Question

I am having trouble creating a tree hierarchy in Python 3. I'd like to be able to do this without using classes.

The data I need to start with is not in order and in the format ['ID','Parent']:

data=[['E1', 'C1'],['C1', 'P1'],['P1', 'R1'],['E2', 'C2'],['C2', 'P2'],['P2', 'R1'],['C3', 'P2'],['E3', 'C4'],['C4', 'P3'],
  ['P3', 'R2'],['C5', 'P3'],['E4', 'C6'],['C6', 'P4'], ['P4', 'R2'],['E5', 'C7'],['C7', 'P5'],['P5', 'R3'],['E6', 'C9'],['C9', 'P6'],['P6', 'R3'],
  ['C8', 'P6'],['E7', 'C10'],['C10', 'P7'],['P7', 'R4'],['C11', 'P7'],['E8', 'C12'],['C12', 'P8'],['P8', 'R4']]

I want to create the (Tree) dictionary variable without the use of classes and end up with something like:

Tree={'R1':{'P1':{},'P2':{}},'R2':{}} etc

OR

Tree={'R1':[{'P1':[],'P2':[]}],'R2':[]} etc

Obviously R1 and R2 have more children than that but perhaps that's what the Tree structure would look like?

Answer 1

You can simply iterate over every child,parent tuple, create dictionary that maps the id's of the child and the parent to a list that contains the children of these elements. We keep doing this until we are done.

roots = set()
mapping = {}
for child,parent in data:
    childitem = mapping.get(child,None)
    if childitem is None:
        childitem =  {}
        mapping[child] = childitem
    else:
        roots.discard(child)
    parentitem = mapping.get(parent,None)
    if parentitem is None:
        mapping[parent] = {child:childitem}
        roots.add(parent)
    else:
        parentitem[child] = childitem

Now that we have done that, roots is a set of the ids of the tree roots: so for each such element we know that there is no id that is a parent. For each id in the roots, we can simply fetch from the mapping and that is a dictionary of the structure {'childid':child} where childid is the id (here a string) and child is again a dictionary of that form.

So you can print them like:

for root in roots:
    print(mapping[root])

So in your case, the tree is:

tree = { id : mapping[id] for id in roots }

For your sample data, it generates:

>>> tree
{'R1': {'P1': {'C1': {'E1': {}}}, 'P2': {'C2': {'E2': {}}, 'C3': {}}}, 'R2': {'P4': {'C6': {'E4': {}}}, 'P3': {'C5': {}, 'C4': {'E3': {}}}}, 'R3': {'P6': {'C8': {}, 'C9': {'E6': {}}}, 'P5': {'C7': {'E5': {}}}}, 'R4': {'P8': {'C12': {'E8': {}}}, 'P7': {'C11': {}, 'C10': {'E7': {}}}}}