Dictionaries of dictionaries merge

Question

I need to merge multiple dictionaries, here's what I have for instance:

dict1 = {1:{"a":{A}}, 2:{"b":{B}}}

dict2 = {2:{"c":{C}}, 3:{"d":{D}}

With A B C and D being leaves of the tree, like {"info1":"value", "info2":"value2"}

There is an unknown level(depth) of dictionaries, it could be {2:{"c":{"z":{"y":{C}}}}}

In my case it represents a directory/files structure with nodes being docs and leaves being files.

I want to merge them to obtain:

 dict3 = {1:{"a":{A}}, 2:{"b":{B},"c":{C}}, 3:{"d":{D}}}

I'm not sure how I could do that easily with Python.

Answer 1

Since dictviews support set operations, I was able to greatly simplify jterrace's answer.

def merge(dict1, dict2):
    for k in dict1.keys() - dict2.keys():
        yield (k, dict1[k])

    for k in dict2.keys() - dict1.keys():
        yield (k, dict2[k])

    for k in dict1.keys() & dict2.keys():
        yield (k, dict(merge(dict1[k], dict2[k])))

Any attempt to combine a dict with a non dict (technically, an object with a 'keys' method and an object without a 'keys' method) will raise an AttributeError. This includes both the initial call to the function and recursive calls. This is exactly what I wanted so I left it. You could easily catch an AttributeErrors thrown by the recursive call and then yield any value you please.

Answer 2

Easiest way i can think of is :

#!/usr/bin/python

from copy import deepcopy
def dict_merge(a, b):
    if not isinstance(b, dict):
        return b
    result = deepcopy(a)
    for k, v in b.iteritems():
        if k in result and isinstance(result[k], dict):
                result[k] = dict_merge(result[k], v)
        else:
            result[k] = deepcopy(v)
    return result

a = {1:{"a":'A'}, 2:{"b":'B'}}
b = {2:{"c":'C'}, 3:{"d":'D'}}

print dict_merge(a,b)

Output:

{1: {'a': 'A'}, 2: {'c': 'C', 'b': 'B'}, 3: {'d': 'D'}}

Answer 3

this is actually quite tricky - particularly if you want a useful error message when things are inconsistent, while correctly accepting duplicate but consistent entries (something no other answer here does....)

assuming you don't have huge numbers of entries a recursive function is easiest:

def merge(a, b, path=None):
    "merges b into a"
    if path is None: path = []
    for key in b:
        if key in a:
            if isinstance(a[key], dict) and isinstance(b[key], dict):
                merge(a[key], b[key], path + [str(key)])
            elif a[key] == b[key]:
                pass # same leaf value
            else:
                raise Exception('Conflict at %s' % '.'.join(path + [str(key)]))
        else:
            a[key] = b[key]
    return a

# works
print(merge({1:{"a":"A"},2:{"b":"B"}}, {2:{"c":"C"},3:{"d":"D"}}))
# has conflict
merge({1:{"a":"A"},2:{"b":"B"}}, {1:{"a":"A"},2:{"b":"C"}})

note that this mutates a - the contents of b are added to a (which is also returned). if you want to keep a you could call it like merge(dict(a), b).

agf pointed out (below) that you may have more than two dicts, in which case you can use:

reduce(merge, [dict1, dict2, dict3...])

where everything will be added to dict1.

[note - i edited my initial answer to mutate the first argument; that makes the "reduce" easier to explain]

ps in python 3, you will also need from functools import reduce

Answer 4

This should help in merging all items from dict2 into dict1:

for item in dict2:
    if item in dict1:
        for leaf in dict2[item]:
            dict1[item][leaf] = dict2[item][leaf]
    else:
        dict1[item] = dict2[item]

Please test it and tell us whether this is what you wanted.

EDIT:

The above mentioned solution merges only one level, but correctly solves the example given by OP. To merge multiple levels, the recursion should be used.

Answer 5

Based on @andrew cooke. This version handles nested lists of dicts and also allows the option to update the values

def merge(a, b, path=None, update=True):
    "http://stackoverflow.com/questions/7204805/python-dictionaries-of-dictionaries-merge"
    "merges b into a"
    if path is None: path = []
    for key in b:
        if key in a:
            if isinstance(a[key], dict) and isinstance(b[key], dict):
                merge(a[key], b[key], path + [str(key)])
            elif a[key] == b[key]:
                pass # same leaf value
            elif isinstance(a[key], list) and isinstance(b[key], list):
                for idx, val in enumerate(b[key]):
                    a[key][idx] = merge(a[key][idx], b[key][idx], path + [str(key), str(idx)], update=update)
            elif update:
                a[key] = b[key]
            else:
                raise Exception('Conflict at %s' % '.'.join(path + [str(key)]))
        else:
            a[key] = b[key]
    return a

Answer 6

If you have an unknown level of dictionaries, then I would suggest a recursive function:

def combineDicts(dictionary1, dictionary2):
    output = {}
    for item, value in dictionary1.iteritems():
        if dictionary2.has_key(item):
            if isinstance(dictionary2[item], dict):
                output[item] = combineDicts(value, dictionary2.pop(item))
        else:
            output[item] = value
    for item, value in dictionary2.iteritems():
         output[item] = value
    return output