Python: flatten nested dict

flatten_nested_dict.py

def flatten(dd: dict, *, separator: str = '_') -> dict:
    """
    Flatten nested dicts deeply.
    Nested keys must be `str` and will be concatenated with a separator.
    Example:
        >>> flatten({'flat': 1, 'a': {'nested': {'key': 2}}})
        {'flat': 1, 'a_nested_key': 2}
    Note:
        De-nesting can lead to new key collisions and data loss:
        >>> flatten({'a': {'b': 111}, 'a_b': 2})
        {'a_b': 2}
    """
    return dict(_flatten_dict_items(dd.items(), separator))

def _flatten_dict_items(items, separator='_'):
    for key, value in items:
        if not isinstance(value, dict):
            yield key, value
        else:
            yield from (
                (key + separator + inner_key, inner_val)
                for inner_key, inner_val in _flatten_dict_items(value.items(), separator)
            )

Interesting edge cases

Input	Output
{'': {'': 1}}	{'_': 1}
{'': {'a': {'': 2}}}	{'_a_': 2}
({'a': {'': {'c': 3}}}	{'a__c': 3}
{'a': {'b': {}}}	{}
"NOT_A_DICT"	AttributeError