Recently, I had a problem. I only had a list of child to parent relations. I needed to create a nested list structure. This is how I did it.

nested_list.py

import collections


def create_tree(edges):
    '''
    This parses the edges and creates a nested dictionary
    '''

    trees = collections.defaultdict(dict)

    for child, parent in edges:
        trees[parent][child] = trees[child]

    children, parents = zip(*edges)
    roots = set(parents).difference(children)

    return {root: trees[root] for root in roots}


def nested_list(tree, n_list=[], level=0):
    '''
    This is where we create our nested list. It's a recursive function.
    '''
    for key, value in tree.items():
        idx = len(n_list)
        n_list.append(
            {'id': key, 'nodes': []}
        )
        nested_list(
            value, n_list=n_list[idx]['nodes'], level=level + 1
        )
    return n_list


def main():
    # Test edges
    edges = [(1, 2), (2, 3), (1, 3), (9, 12), (1, 12)]
    # Create the nested tree
    tree = create_tree(edges)
    # Now create the nest list
    n_list = nested_list(tree)


if __name__ == '__main__':
    main()