Python3 dictionary comprehension with sub-dictionary upacking?

Suppose one has a dictionary, root which consists of key:value pairs, where some values are themselves dictionaries.

root

Can one (and if so, how) unpack these sub dictionaries via dictionary comprehension?

e.g.

k: v if type(v) is not dict else **v for k, v in root.items()


example:

root = 'a': 1, 'b': 'c': 2, 'd': 3


result = 'a': 1, 'c': 2, 'd': 3


_k: _v for k,v in root.items() for _k, _v in (v if isinstance(v, dict) else k: v).items()

3 Answers
3

I guess I should post as an answer with a more broad explanation to help you as it is a little bit different to other existing questions


_k: _v
for k, v in root.items()
for _k, _v in ( # here I create a dummy dictionary if non exists
v if isinstance(v, dict) else k: v
).items() # and iterate that



The key part of understanding is that you need consistent and generic logic for the comprehension to work.

You can do this by creating dummy nested dictionaries where they don't previously exist using v if isinstance(v, dict) else k: v

v if isinstance(v, dict) else k: v

Then this is a simple nested dictionary unpacking exercise.

To help in your future comprehensions I would recommend writing the code out e.g.

res = dict()
for k,v in root.items():
d = v if isinstance(v, dict) else k: v
for _k, _v in d.items():
res[_k] = _v


and work backwards from this

Useful references

If you have several levels of nested dictionaries, I suggest you the following solution based on a recursive function:

def flatten(res, root):
for k,v in root.items():
if isinstance(v, dict):
flatten(res, v)
else:
res[k] = v

root = 'a': 1, 'b': 'c': 2, 'd': 'e': 5, 'f': 6

result =
flatten(result, root)

print(result) # 'a': 1, 'c': 2, 'e': 5, 'f': 6


Here is a recursive solution. In the function _flatten_into_kv_pairs, we iterate through the key and values pair and yield those keys/values if the value is a not a dictionary. If it is, then we recursively call _flatten_into_kv_pairs with the yield from construct. The function flatten_dict is just a shell which turns sequence of key/value pairs back into a dictionary.

_flatten_into_kv_pairs

_flatten_into_kv_pairs

yield from

flatten_dict

def _flatten_into_kv_pairs(dict_object):
for k, v in dict_object.items():
if isinstance(v, dict):
yield from _flatten_into_kv_pairs(v)
else:
yield k, v


def flatten_dict(dict_object):
return dict(_flatten_into_kv_pairs(dict_object))


root = 'a': 1, 'b': 'c': 2, 'd': 3, 'e': 'f': 4, 'g': 5
print(flatten_dict(root))


Output:

'a': 1, 'c': 2, 'd': 3, 'f': 4, 'g': 5


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