Iterate over dictionaries with variable keys in predefined order

ordered.py

"""Iterate over dicts with variable keys in predefined order."""

from collections.abc import Iterable
from typing import Self


class KeyOrder(dict):
    """Key -> index mapping for iterating over dicts (unknown keys last)."""

    @classmethod
    def fromlist(cls, keys: Iterable[str]) -> Self:
        """Define the order of keys as given."""
        return cls((k, i) for i, k in enumerate(keys))

    def itersorted(self, dct):
        """Iterate over dct (key, value) pairs in the defined order."""
        for key in sorted(dct, key=self._itersorted_key):
            yield key, dct[key]

    def _itersorted_key(self, key):
         return self[key], key

    _missing = float('inf')

    def __missing__(self, key) -> float:
        return self._missing


if __name__ == '__main__':
    keyorder = KeyOrder.fromlist(['spam', 'eggs'])
    spam = {'spam': 'spam', 'ham': 'ham', 'eggs': 'eggs'}

    for k, v in keyorder.itersorted(spam):
        print(k)