Python: create a nested dictionary from a list of parent child values

Question

Here is the input:

list_child_parent= [
    #first value is child, second is parent
    (0, 1),
    (1, 3),
    (8, 7),
    (3, 6),
    (4, 3),
    (5, 3)
]

The output needs to create a nested dictionary tree using these values. The tree will never be more than 6 levels deep.

For example:

output_dict = {
    6: {3: {1: {0: {}}, 4: {}, 5: {}}}, 7: {8: {}}
}

I have spent two days trying to accomplish this. I have tried writing functions to find where the key is in the tree and then add the new key after it, but I cannot produce code that can continue more than 3 levels. It's baffling and I feel that there is probably a standard library that can do this.

My experience level is low.

Answer 1

Not pretty and probably not Pythonic, but it should get you going:

#!/usr/bin/env python3

def make_map(list_child_parent):
    has_parent = set()
    all_items = {}
    for child, parent in list_child_parent:
        if parent not in all_items:
            all_items[parent] = {}
        if child not in all_items:
            all_items[child] = {}
        all_items[parent][child] = all_items[child]
        has_parent.add(child)

    result = {}
    for key, value in all_items.items():
        if key not in has_parent:
            result[key] = value
    return result

if __name__ == '__main__':
    list_child_parent = [
        #first value is child, second is parent
        (0, 1),
        (1, 3),
        (8, 7),
        (3, 6),
        (4, 3),
        (5, 3)
    ]

    actual = make_map(list_child_parent)

    expected = {
        6: {
            3: {
                1: {
                    0: {}
                },
                4: {},
                5: {}
            }
        },
        7: {
            8: {}
        }
    }
    print('OK' if expected == actual else 'FAIL')