The best way to merge multi-nested dictionaries in Python 2.7

Question

I have two nested dictionaries and I want to merge them into one (where second dict overrides first dict values). I saw a lot of beautiful solutions for merging "flat" (not nested) dictionaries, e.g.:

dict_result = dict1.copy()
dict_result.update(dict2)

or

dict_result = dict(dict1.items() + dict2.items())

or (my favorit one)

dict_result = dict(d1,**d2)

but couldn't find the most efficient way to merge multi-nested dicts.

I'm trying to avoid recursion. What is your proposition?

Answer 1

Unless the depth of the dictionaries to merge is strictly limited, there's no way to avoid recursion.¹⁾ Also, there's no bultin or library function to do this (that is, none that I know of), but it's actually not all that hard. Something like this should do:

def merge(d1, d2):
    for k in d2:
        if k in d1 and isinstance(d1[k], dict) and isinstance(d2[k], dict):
            merge(d1[k], d2[k])
        else:
            d1[k] = d2[k]   

What this does: It iterates the keys in d2 and if the key can also be found in d1 and both are dictionaries, merge those sub-dictionaries, otherwise overwrite the value in d1 with that from d2. Note that this changes d1 and its sub-dictionaries in place, so you might want to deep-copy it before.

Or use this version to create a merged copy:

def merge_copy(d1, d2):
    return {k: merge_copy(d1[k], d2[k]) if k in d1 and isinstance(d1[k], dict) and isinstance(d2[k], dict) else d2[k] for k in d2}

Example:

>>> d1 = {"foo": {"bar": 23, "blub": 42}, "flub": 17}
>>> d2 = {"foo": {"bar": 100}, "flub": {"flub2": 10}, "more": {"stuff": 111}}
>>> merge(d1, d2)
>>> print d1
{'foo': {'bar': 100, 'blub': 42}, 'flub': {'flub2': 10}, 'more': {'stuff': 111}}

---

¹⁾ You can make it iterative, using a stack, but this will only make things more complicated and should only be done to avoid problems with maximum recursive depth.

Answer 2

Modified version of the above merge_copy function for dicts that can be thought of as merging parent and child where you want the parent to inherit all the values from child to create a new dict.

def merge_copy(child, parent):
    '''returns parent updated with child values if exists'''
    d = {}
    for k in parent:
        if k in child and isinstance(child[k], dict) and isinstance(parent[k], dict):
            v = merge_copy(child[k], parent[k])
        elif k in child:
            v = child[k]
        else:
            v = parent[k]
        d[k] = v
    return d