Broken example of a protocol registry

scratchpad.py

"""A broken attempt at a Protocol-based registry.

I have a system with replaceable implementations. A ``Heading``
might be asked for on a page, but which one you get depends
on some criteria.

mypy doesn't seem to like using Protocols at runtime. This
small module has 7 errors.
"""
from dataclasses import dataclass
from typing import Protocol, TypeVar


class Heading(Protocol):
    full_name: str


@dataclass()
class DataclassHeading:
    full_name: str = "Dataclass Heading"


class PlainClassHeading:
    full_name: str

    def __init__(self, full_name: str = "Plain Class Heading"):
        self.full_name = full_name


# Problems begin. Is ``TypeVar`` even the right concept?
T = TypeVar("T")
#
implementations: dict[Protocol[T], T] = dict(
    Heading=[DataclassHeading, PlainClassHeading],
)


def construct(protocol: Protocol[T], choice: int) -> T:
    target = implementations[protocol][choice]
    return target()


dh = construct(Heading, 0)
pch = construct(Heading, 1)