Merge nested dictionaries, by nested keys?

Question

I have several dictionaries with different and common keys, plus different and common keys in the nested dictionary. Below is a simplified example, the actual dictionaries have thousands of keys.

{1:{"Title":"Chrome","Author":"Google","URL":"http://"}}
{1:{"Title":"Chrome","Author":"Google","Version":"7.0.577.0"}}
{2:{"Title":"Python","Version":"2.5"}}

Which I'd like to merge into a single dictionary.

{1:{"Title":"Chrome","Author":"Google","URL":"http://","Version":"7.0.577.0"},
 2:{"Title":"Python","Version":"2.5"}}

I can iterate over both dictionaries, compare keys and update the nested dictionaries, but there is probably a more efficient, or pythonic, way to do this. If not, which is the most efficient?

Values of the nested dictionary need not be compared.

Horseman

If you're really mapping sequential integer keys, wouldn't it make more sense to produce a list as the output? – Tinkle

Lots of discussion and tips here as well: #39487 – Cutshall

Seems very unrealistic that each of the "several dictionaries" has exactly one key/value pair in it -- makes one wonder why they're dictionaries at all when a simple tuple or list with two items in it would work just as well. – Padding

Answer 1

from collections import defaultdict

mydicts = [
   {1:{"Title":"Chrome","Author":"Google","URL":"http://"}},
   {1:{"Title":"Chrome","Author":"Google","Version":"7.0.577.0"}},
   {2:{"Title":"Python","Version":"2.5"}},
]

result = defaultdict(dict)

for d in mydicts:
    for k, v in d.iteritems():
        result[k].update(v)

print result

---

defaultdict(<type 'dict'>, 
    {1: {'Version': '7.0.577.0', 'Title': 'Chrome', 
         'URL': 'http://', 'Author': 'Google'}, 
     2: {'Version': '2.5', 'Title': 'Python'}})

Shieh

Minor tweak: The second for could be replaced with k,v = next(d.iteritems()) and the next line dedented. Not really much difference, but it would look faster. ;-) – Padding

@martineau: it would also look less useful: next() only works on python 2.6+. Also the way I wrote accepts more than one key on each dict, or empty dict without choking or doing the wrong thing – Shieh

Well, you're correct about next() not being available prior to v2.6 -- when writing it I forgot the OP wanted a v2.5 compatible solution. However it's arguable whether accepting ill-formed nested dicts with more than one key would be desirable, and lastly, next() can handle empty dicts without a problem. All things considered, I still believe my comment is a valid observation to those out there using at least v2.6 of Python. – Padding

Answer 2

From your example, looks like you can do something like:

from collections import defaultdict
mydict = defaultdict(dict)
for indict in listofdicts:
    k, v = indict.popitem()
    mydict[k].update(v)

Tinkle

Answer 3

Try with a NestedDict. First install ndicts

pip install ndicts

Then

from ndicts.ndicts import NestedDict

my_dicts = [
   {1:{"Title":"Chrome","Author":"Google","URL":"http://"}},
   {1:{"Title":"Chrome","Author":"Google","Version":"7.0.577.0"}},
   {2:{"Title":"Python","Version":"2.5"}},
]
nd = NestedDict()

for my_dict in my_dicts:
    nd.update(NestedDict(my_dict))

To get the result as a dictionary

>>> nd.to_dict()
{1: {'Title': 'Chrome', 'Author': 'Google', 'Version': '7.0.577.0', 'URL': 'http://'}, 
 2: {'Title': 'Python', 'Version': '2.5'}}

Nethermost

Answer 4

This approach is much faster

my_dicts = [   {1:{"Title":"Chrome","Author":"Google","URL":"http://"}},   {1:{"Title":"Chrome","Author":"Google","Version":"7.0.577.0"}},   {2:{"Title":"Python","Version":"2.5"}},]
result = {}
for d in my_dicts:
    for k, v in d.items():
        result.setdefault(k, {}).update(v)
print(result)

Isolt