For the situation, where you have 2 yaml files, one is older, but fits in 50% of bigger, and bigger have just new keys, then you don't want to do something from beginning, and consider the script that will just join 2 files, but take objects that exists from the bigger file, uff.

yamlunion.py

import ruamel.yaml as yamler
import ruamel
import sys
def main():
    resultDict = dict()
    if(len(sys.argv) < 4):
        print("USAGE: script biggerYaml smallerYaml resultfile")
        print("If entry is in smaller yaml - it will go to result")
        print("If entry is not in smaller yaml - it will take it from biggerYaml")
        sys.exit(1)

    firstYAML = yamler.YAML()
    a = open(sys.argv[1])
    firsts = firstYAML.load(a)
    a.close()
    a = open(sys.argv[2])
    second = firstYAML.load(a)
    a.close()

    result = process(firsts, second, resultDict)
    with open(sys.argv[3], "w") as fp:
        firstYAML.dump(result, fp)

def process(entry, entrysmaller, result):
    types = type(entry)
    if types == ruamel.yaml.comments.CommentedMap:
        for item in entry:
            typeOfItem = type(entry[item])
            if(typeOfItem == str):
                if item in entrysmaller:
                    result[item] = entrysmaller[item]
                else:
                    result[item] = entry[item]
            elif(typeOfItem == ruamel.yaml.comments.CommentedMap):
                if item not in entrysmaller:
                    result[item] = entry[item]
                else:
                    result[item] = dict()
                    process(entry[item], entrysmaller[item], result[item])
            elif typeOfItem == ruamel.yaml.scalarstring.LiteralScalarString:
                if item in entrysmaller:
                    result[item] = entrysmaller[item]
                else:
                    result[item] = entry[item]
            else:
                print("WARNING...")
                print("NOT SUPPORTED CLASS:", typeOfItem)
                print("NOT SUPPORTED VALUE:", item)
                print("...ENDWARNING")

    return result


main()