Does the namedtuple not suffice? Apologies if I'm being dense.

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It's not an immutable dict type (for one thing, this has linear lookup, the BTree would be better) but it's fun:

https://stackoverflow.com/questions/13708701/how-to-implemen...

In Python:

    from functools import partial

    def empty_dict(key):
        raise KeyError

    def _dict_add(dictionary, key, value, lookup):
        return value if key == lookup else dictionary(lookup)

    def dict_add(d, key, value):
        return partial(_dict_add, d, key, value)


    d = empty_dict
    d = dict_add(d, 'key0', 23)
    d = dict_add(d, 'key1', 18)

    >>> d('key0')
    23
    >>> d('key1')
    18
    >>> d('keyn')
    Traceback (most recent call last):
      File "<pyshell#8>", line 1, in <module>
        d('keyn')
      File "/usr/home/sforman/tmp_fn_dict.py", line 7, in _dict_add
        return value if key == lookup else dictionary(lookup)
      File "/usr/home/sforman/tmp_fn_dict.py", line 7, in _dict_add
        return value if key == lookup else dictionary(lookup)
      File "/usr/home/sforman/tmp_fn_dict.py", line 4, in empty_dict
        raise KeyError
    KeyError

> objects built with pydantic and type checked with mypy

FWIW, after ~15 years of professional Python development, I'm not into types [in Python]. My attitude is that if you really need them you should switch to e.g. OCaml or something that does them right. I love Python but I wouldn't use it anymore for anything other than scripts and maybe prototyping.