Non-recursive flatten nested dictionaries in Python3

flatten_dict.py

# Non-recursive flatten nested dictionaries in Python3
# Source: https://codereview.stackexchange.com/a/173483/173155

from itertools import chain, starmap


def flatten_dict(dictionary):
    """Flatten a nested dictionary structure"""

    def unpack(parent_key, parent_value):
        """Unpack one level of nesting in a dictionary"""
        try:
            items = parent_value.items()
        except AttributeError:
            # parent_value was not a dict, no need to flatten
            yield (parent_key, parent_value)
        else:
            for key, value in items:
                yield (parent_key + (key,), value)

    # Put each key into a tuple to initiate building a tuple of subkeys
    dictionary = {(key,): value for key, value in dictionary.items()}

    while True:
        # Keep unpacking the dictionary until all value's are not dictionary's
        dictionary = dict(chain.from_iterable(starmap(unpack, dictionary.items())))
        if not any(isinstance(value, dict) for value in dictionary.values()):
            break

    return dictionary