json.loads allows duplicate keys in a dictionary, overwriting the first value

Question

>>> raw_post_data = request.raw_post_data
>>> print raw_post_data
{\"group\":{\"groupId\":\"2\", \"groupName\":\"GroupName\"}, \"members\":{\"1\":{         


        

Answer 1

This is a linter-fixed and type-annotated version of the answer by jfs. Issues highlighted by various linters were addressed. It is also modernized for Python 3.6+ to use f-strings.

import json
from typing import Any, Dict, Hashable, List, Tuple

def raise_on_duplicate_keys(ordered_pairs: List[Tuple[Hashable, Any]]) -> Dict:
    """Raise ValueError if a duplicate key exists in provided ordered list of pairs, otherwise return a dict."""
    dict_out = {}
    for key, val in ordered_pairs:
        if key in dict_out:
            raise ValueError(f'Duplicate key: {key}')
        else:
            dict_out[key] = val
    return dict_out

json.loads('{"x": 1, "x": 2}', object_pairs_hook=raise_on_duplicate_keys)

ordered_pairs above is a list of tuples, with each tuple having a key and a value. Refer to the docs for object_pairs_hook.