Converting list to nested dictionary

Question

How can I convert a list into nested `dictionary\'?

For example:

l = [1, 2, 3, 4] 

I\'d like to convert it to a dictio

Answer 1

For that reverse the list, then start creating the empty dictionary element.

l = [1, 2, 3, 4]
d = {}
for i in reversed(l):
    d = {i: d}

>>> print(d)
{1: {2: {3: {4: {}}}}}

Answer 2

You could also use functools.reduce for this.

reduce(lambda cur, k: {k: cur}, reversed(l), {})

Demo

>>> from functools import reduce
>>> l = [1, 2, 3, 4]

>>> reduce(lambda cur, k: {k: cur}, reversed(l), {})
{1: {2: {3: {4: {}}}}}

---

The flow of construction looks something like

{4: {}} -> {3: {4: {}} -> {2: {3: {4: {}}}} -> {1: {2: {3: {4: {}}}}}

as reduce traverses the reverse iterator making a new single-element dict.

Answer 3

You can do something like this:

l = [1,2,3,4]
d = {}

for i in l[::-1]:
    d = {i: d}

print(d)

{1: {2: {3: {4: {}}}}} [Finished in 0.4s]

Answer 4

Here is an abstraction. Uses for setdefault are typically overshadowed by defaultdict, but here is an interesting application if you have one or more lists (iterables):

def make_nested_dict(*iterables):
    """Return a nested dictionary."""
    d = {}
    for it in iterables:
        temp = d
        for i in it:
            temp = temp.setdefault(i, {})
    return d

make_nested_dict([1, 2, 3, 4])
# {1: {2: {3: {4: {}}}}}

make_nested_dict([1, 2, 3, 4], [5, 6])
# {1: {2: {3: {4: {}}}}, 5: {6: {}}}

---

Nested Branches

Unlike defaultdict, this technique accepts duplicate keys by appending to existing "branches". For example, we will append a new 7 → 8 branch at the third level of the first (A) branch:

                       A         B           C            
make_nested_dict([1, 2, 3, 4], [5, 6], [1, 2, 7, 8])
# {1: {2: {3: {4: {}}, 7: {8: {}}}}, 5: {6: {}}}

Visually:

1 → 2 → 3 → 4   (A)            5 → 6   (B)
       \ 
        7 → 8   (C)