How to raise error if duplicates keys in dictionary

Question

I try to raise an error if the user enter a duplicate key in a dictionary. The dictionary is in a file and the user can edit the file manually.

Example:

dico= {'root':{
                'a':{'some_key':'value',...},
                'b':{'some_key':'value',...},
                'c':{'some_key':'value',...},
                ...

                'a':{'some_key':'value',...},
              }
      }

the new key 'a' already exist...

How can I test dico and warn the user when I load dico from the file?

Answer 1

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!).

Answer 2

If you want to ensure that an error is raised during dict construction with duplicate keys, just leverage Python's native keyword argument checking:

> dict(a={}, a={})
SyntaxError: keyword argument repeated

Unless I'm missing something, there is no need to subclass dict.