如何在 Python 中迭代 N 级嵌套字典？

Question

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Looking for a (recursive?) way of iterating through the keys of nested dictionaries without specifying the number of levels a priori or hardcoding in multiple levels.

I find myself making multilevel dictionaries quite a bit. I always have to write very verbose code to iterate through all the levels of the dictionaries with a lot of temporary variables.

Is there a way to generalize this function to iterate through multiple levels instead of hardcoding in and manually specifying how many levels there are?

def iterate_multilevel_dictionary(d, number_of_levels):
    # How to auto-detect number of levels? 
    # number_of_levels = 0
    if number_of_levels == 1:
        for k1, v1 in d.items():
            yield k1, v1
    if number_of_levels == 2:
        for k1, v1 in d.items():
            for k2, v2 in v1.items():
                yield k1, k2, v2
    if number_of_levels == 3:
        for k1, v1 in d.items():
            for k2, v2 in v1.items():
                for k3, v3 in v2.items():
                    yield k1, k2, k3, v3
                    
# Level 1
d_level1 = {"a":1,"b":2,"c":3}
for items in iterate_multilevel_dictionary(d_level1, number_of_levels=1):
    print(items)
# ('a', 1)
# ('b', 2)
# ('c', 3)

# Level 2
d_level2 = {"group_1":{"a":1}, "group_2":{"b":2,"c":3}}
for items in iterate_multilevel_dictionary(d_level2, number_of_levels=2):
    print(items)
#('group_1', 'a', 1)
#('group_2', 'b', 2)
#('group_2', 'c', 3)

# Level 3
d_level3 = {"collection_1":d_level2}
for items in iterate_multilevel_dictionary(d_level3, number_of_levels=3):
    print(items)
# ('collection_1', 'group_1', 'a', 1)
# ('collection_1', 'group_2', 'b', 2)
# ('collection_1', 'group_2', 'c', 3)

Answer 1

I've written this after I saw @VoNWooDSoN's answer. I turned it into an iterator instead of printing inside the function and a little bit of changes to make it more readable. So see his original answer here.

def flatten(d, base=()):
    for k, v in d.items():
        if isinstance(v, dict):
            yield from flatten(v, base + (k,))
        else:
            yield base + (k, v)

1- yielding instead of printing.

2- isinstance() instead of type so that subclasses of dict can also work. You could also use MutableMapping from typing module instead of dict to make it more generic.

3- IMO , getting (k, v) pairs from .items() is much more readable than k and d[k] .

More generic ?

Do you wanna expand this to even more generic which CAN (not have to, like the solution in the OP) accept the number of depths just in case?

Consider these examples:

d_level1 = {"a": 1, "b": 2, "c": 3}
d_level2 = {"group_1": {"a": 1}, "group_2": {"b": 2, "c": 3}}
d_level3 = {"collection_1": d_level2}

for items in flatten(d_level3):
    print(items)
print('------------------------------')
for items in flatten(d_level3, depth=0):
    print(items)
print('------------------------------')
for items in flatten(d_level3, depth=1):
    print(items)
print('------------------------------')
for items in flatten(d_level3, depth=2):
    print(items)

output:

('collection_1', 'group_1', 'a', 1)
('collection_1', 'group_2', 'b', 2)
('collection_1', 'group_2', 'c', 3)
------------------------------
('collection_1', {'group_1': {'a': 1}, 'group_2': {'b': 2, 'c': 3}})
------------------------------
('collection_1', 'group_1', {'a': 1})
('collection_1', 'group_2', {'b': 2, 'c': 3})
------------------------------
('collection_1', 'group_1', 'a', 1)
('collection_1', 'group_2', 'b', 2)
('collection_1', 'group_2', 'c', 3)

depth=None doesn't consider the depth (still works like you want at the first place). But now by specifying depths from 0 to 2 you can see that we are able to iterate how deep we want. here is the code:

def flatten(d, base=(), depth=None):
    for k, v in d.items():
        if not isinstance(v, dict):
            yield base + (k, v)
        else:
            if depth is None:
                yield from flatten(v, base + (k,))
            else:
                if depth == 0:
                    yield base + (k, v)
                else:
                    yield from flatten(v, base + (k,), depth - 1)

Answer 2

Here's a quick and dirty solution for you:

d_level1 = {"a":1,"b":2,"c":3}
d_level2 = {"group_1":{"a":1}, "group_2":{"b":2,"c":3}}
d_level3 = {"collection_1":d_level2}

def flatten(d_in, base=()):
    for k in d_in:
        if type(d_in[k]) == dict:
            flatten(d_in[k], base+(k,))
        else:
            print(base + (k, d_in[k]))

flatten(d_level1)
# ('a', 1)
# ('b', 2)
# ('c', 3)

flatten(d_level2)
#('group_1', 'a', 1)
#('group_2', 'b', 2)
#('group_2', 'c', 3)

flatten(d_level3)
# ('collection_1', 'group_1', 'a', 1)
# ('collection_1', 'group_2', 'b', 2)
# ('collection_1', 'group_2', 'c', 3)

Be aware!! Python has a recursion limit of about 1000! So, when using recursion in python think very carefully what you're trying to do and be prepared to catch a RuntimeError if you call a recursive function like this.

EDIT: With comments I realized that I'd made a mistake where I did not add the key to the level1 dict output and that I was using a mutable structure as a default argument. I added these and parens in the print statement and reposted. The output now matches the OP's desired output and uses better and modern python.

Answer 3

try out this code

it also supports a combination of levels

from typing import List, Tuple


def iterate_multilevel_dictionary(d: dict):
    dicts_to_iterate: List[Tuple[dict, list]] = [(d, [])]
    '''
    the first item is the dict object and the second object is the prefix keys 
    '''
    while dicts_to_iterate:
        current_dict, suffix = dicts_to_iterate.pop()
        for k, v in current_dict.items():
            if isinstance(v, dict):
                dicts_to_iterate.append((v, suffix + [k]))
            else:
                yield suffix + [k] + [v]


if __name__ == '__main__':
    d_level1 = {"a": 1, "b": 2, "c": 3}
    print(f"test for {d_level1}")
    for items in iterate_multilevel_dictionary(d_level1):
        print(items)
    d_level2 = {"group_1": {"a": 1}, "group_2": {"b": 2, "c": 3}}
    print(f"test for {d_level2}")
    for items in iterate_multilevel_dictionary(d_level2):
        print(items)

    d_level3 = {"collection_1": d_level2}
    print(f"test for {d_level3}")
    for items in iterate_multilevel_dictionary(d_level3):
        print(items)

    d_level123 = {}
    [d_level123.update(i) for i in [d_level1, d_level2, d_level3]]
    print(f"test for {d_level123}")
    for items in iterate_multilevel_dictionary(d_level123):
        print(items)

the outputs is:

test for {'a': 1, 'b': 2, 'c': 3}
['a', 1]
['b', 2]
['c', 3]
test for {'group_1': {'a': 1}, 'group_2': {'b': 2, 'c': 3}}
['group_2', 'b', 2]
['group_2', 'c', 3]
['group_1', 'a', 1]
test for {'collection_1': {'group_1': {'a': 1}, 'group_2': {'b': 2, 'c': 3}}}
['collection_1', 'group_2', 'b', 2]
['collection_1', 'group_2', 'c', 3]
['collection_1', 'group_1', 'a', 1]
test for {'a': 1, 'b': 2, 'c': 3, 'group_1': {'a': 1}, 'group_2': {'b': 2, 'c': 3}, 'collection_1': {'group_1': {'a': 1}, 'group_2': {'b': 2, 'c': 3}}}
['a', 1]
['b', 2]
['c', 3]
['collection_1', 'group_2', 'b', 2]
['collection_1', 'group_2', 'c', 3]
['collection_1', 'group_1', 'a', 1]
['group_2', 'b', 2]
['group_2', 'c', 3]
['group_1', 'a', 1]

using recursion is another approach but I thought writing without recursion is more challenging and more efficient :)