How to raise error if duplicates keys in dictionary

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Write a subclass of dict, override __setitem__ such that it throws an error when replacing an existing key; rewrite the file to use your new subclass’s constructor instead of the default dict built-ins.

import collections

class Dict(dict):
    def __init__(self, inp=None):
        if isinstance(inp,dict):
            super(Dict,self).__init__(inp)
        else:
            super(Dict,self).__init__()
            if isinstance(inp, (collections.Mapping, collections.Iterable)): 
                si = self.__setitem__
                for k,v in inp:
                    si(k,v)

    def __setitem__(self, k, v):
        try:
            self.__getitem__(k)
            raise ValueError("duplicate key '{0}' found".format(k))
        except KeyError:
            super(Dict,self).__setitem__(k,v)

then your file will have to be written as

dico = Dict(
    ('root', Dict(
        ('a', Dict(
            ('some_key', 'value'),
            ('another_key', 'another_value')
        ),
        ('b', Dict(
            ('some_key', 'value')
        ),
        ('c', Dict(
            ('some_key', 'value'),
            ('another_key', 'another_value')
        ),

        ....
    )
)

using tuples instead of dicts for the file import (written using the {} notation, it would use the default dict constructor, and the duplicates would disappear before the Dict constructor ever gets them!).

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