Recursive function to create hierarchy from flat dictionary of {string : list} pairs

Question

I need a function to take this dictionary of lists and produce a hierarchical dictionary. I always have the root key so the function can be called with the root key as an argument. There are no circular references. If an item has no children it should return 'DATA'.

I am almost there but in my result those keys that have a dictionary as child node end up added a second time as a child to itself.

flat = {'g': ['h', 'i', 'j'],
        'b': ['e', 'f', 'g', 'm'],
        'a': ['b', 'c', 'd'],
        'm': ['n', 'o', 'p']}


what_the_result_should_be = {'a': {'c': 'DATA','d': 'DATA', 'b': {'m':{'n': 'DATA', 'o': 'DATA', 'p': 'DATA'}, 'e': 'DATA','f': 'DATA','g':{'h': 'DATA', 'i': 'DATA', 'j': 'DATA'}}}}

def walk(d, node):
    if d.get(node, None):
        return {node : {child : walk(d, child) for child in d[node]}}
    else:
        return 'DATA'

what_my_attempt_produces = walk(flat, 'a')

Ajax1234 · Accepted Answer · 2019-07-12 15:16:27Z

2

You can use recursion:

flat = {'g': ['h', 'i', 'j'], 'b': ['e', 'f', 'g', 'm'], 'a': ['b', 'c', 'd'], 'm': ['n', 'o', 'p']}
def create(a):
   r = {}
   for i in flat[a]:
     if i not in flat:
       r[i] = 'DATA'
     else:
       r.update(create(i))
   return {a:r}

result = create('a')
print(what_the_result_should_be == result)

Output:

True

Output:

{'a': {'b': {'e': 'DATA', 'f': 'DATA', 'g': {'h': 'DATA', 'i': 'DATA', 'j': 'DATA'}, 'm': {'n': 'DATA', 'o': 'DATA', 'p': 'DATA'}}, 'c': 'DATA', 'd': 'DATA'}}

edited Jul 12, 2019 at 15:16

answered Jul 12, 2019 at 15:09

Ajax1234

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2 Comments

delica Over a year ago

This produces the some double-child result that I have. The '==' doesn't do a deep comparison here, thus the True.

Ajax1234 Over a year ago

@delica Glad to help!

Green Cloak Guy · Accepted Answer · 2019-07-12 15:24:07Z

1

I would do this in two steps. First, build "dummy" data structures for each of your keys:

dummy = {elem:({} if elem in flat.keys() else 'DATA')
            for v in flat.values() 
            for elem in v}

Then, construct a proper hierarchy using references from "dummy":

result = {}
for key, value in flat.items():
    head = dummy.get(key, {})
    for elem in value:
        head[elem] = dummy[elem]
    if key not in dummy:
        result[key] = head

This yields:

{'a': {'b': {'e': 'DATA',
             'f': 'DATA',
             'g': {'h': 'DATA', 'i': 'DATA', 'j': 'DATA'},
             'm': {'n': 'DATA', 'o': 'DATA', 'p': 'DATA'}},
       'c': 'DATA',
       'd': 'DATA'}}

edited Jul 12, 2019 at 15:24

answered Jul 12, 2019 at 15:10

Green Cloak Guy

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2 Comments

delica Over a year ago

This doesn't run even after I add items() to 'for key, value in flat'

Green Cloak Guy Over a year ago

Ah, it was because a wasn't in any of the values. I used the get() method instead to account for that, sorry

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