How to merge dictionaries of dictionaries?

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}}
         


        

Answer 1

Overview

The following approach subdivides the problem of a deep merge of dicts into:

1. A parameterized shallow merge function merge(f)(a,b) that uses a function f to merge two dicts a and b

2. A recursive merger function f to be used together with merge

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Implementation

A function for merging two (non nested) dicts can be written in a lot of ways. I personally like

def merge(f):
    def merge(a,b): 
        keys = a.keys() | b.keys()
        return {key:f(a.get(key), b.get(key)) for key in keys}
    return merge

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A nice way of defining an appropriate recursive merger function f is using multipledispatch which allows to define functions that evaluate along different paths depending on the type of their arguments.

from multipledispatch import dispatch

#for anything that is not a dict return
@dispatch(object, object)
def f(a, b):
    return b if b is not None else a

#for dicts recurse 
@dispatch(dict, dict)
def f(a,b):
    return merge(f)(a,b)

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Example

To merge two nested dicts simply use merge(f) e.g.:

dict1 = {1:{"a":"A"},2:{"b":"B"}}
dict2 = {2:{"c":"C"},3:{"d":"D"}}
merge(f)(dict1, dict2)
#returns {1: {'a': 'A'}, 2: {'b': 'B', 'c': 'C'}, 3: {'d': 'D'}} 

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Notes:

The advantages of this approach are:

• The function is build from smaller functions that each do a single thing which makes the code simpler to reason about and test

• The behaviour is not hard-coded but can be changed and extended as needed which improves code reuse (see example below).

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Customization

Some answers also considered dicts that contain lists e.g. of other (potentially nested) dicts. In this case one might want map over the lists and merge them based on position. This can be done by adding another definition to the merger function f:

import itertools
@dispatch(list, list)
def f(a,b):
    return [merge(f)(*arg) for arg in itertools.zip_longest(a, b)]