Utility to flatten a dictionary with string values

flattendict.py

from typing import Any, Dict, Union


NestedStringDict = Dict[str, Union[Any, "NestedStringDict"]]


def flatten_dict(nested: NestedStringDict, /, *, sep: str = "_") -> Dict[str, Any]:
    """
    Flatten a nested dictionary with string keys.

    The dictionary keys are strings and the values are either an arbitrary object or
    another dictionary of this kind. The flattened dictionary's keys consist of the
    concatenation of the path of keys (from nested dictionary to nested dictionary)
    that reaches the value.

    :param nested: Dictionary to flatten.
    :param sep: Separator to use when concatenating nested keys.
    :return: A flattened copy of ``d``.

    Example:
        >>> d = {"foo": 42, "bar": {"spam": 12, "eggs": 3}}
        >>> flatten_dict(d)
        {'foo': 42, 'bar_spam': 12, 'bar_eggs': 3}
        
        >>> flatten_dict(d, sep=".")
        {'foo': 42, 'bar.spam': 12, 'bar.eggs': 3}
    """

    def iter_flat_pairs(d, prefix=""):
        for key, value in d.items():
            if isinstance(value, dict):
                yield from iter_flat_pairs(value, prefix=key + sep)
            else:
                yield prefix + key, value

    return dict(iter_flat_pairs(nested))