cfg.py

from typing import TypeVar, Type, Callable, Dict, Any

Attrs = Dict[str, Any]

C = TypeVar('C')

# todo not sure about it, could be overthinking...
# but short enough to change later
# TODO document why it's necessary?
def make_config(cls: Type[C], migration: Callable[[Attrs], Attrs]=lambda x: x) -> C:
    user_config = cls.__base__
    old_props = {
        # NOTE: deliberately use gettatr to 'force' class properties here
        k: getattr(user_config, k) for k in vars(user_config)
    }
    new_props = migration(old_props)
    from dataclasses import fields
    params = {
        k: v
        for k, v in new_props.items()
        if k in {f.name for f in fields(cls)}
    }
    # todo maybe return type here?
    return cls(**params) # type: ignore[call-arg]


F = TypeVar('F')
from contextlib import contextmanager
from typing import Iterator
@contextmanager
def override_config(config: F) -> Iterator[F]:
    '''
    Temporary override for config's parameters, useful for testing/fake data/etc.
    '''
    orig_properties = {k: v for k, v in vars(config).items() if not k.startswith('__')}
    try:
        yield config
    finally:
        # ugh. __dict__ of type objects isn't writable..
        for k, v in orig_properties.items():
            setattr(config, k, v)
        added = {k for k in set(vars(config).keys()).difference(set(orig_properties.keys())) if not k.startswith('__')}
        for k in added:
            delattr(config, k)


# helper for tests? not sure if could be useful elsewhere
@contextmanager
def tmp_config():
    import my.config as C
    with override_config(C):
        yield C # todo not sure?


def test_tmp_config() -> None:
    class extra:
        data_path = '/path/to/data'
    with tmp_config() as c:
        assert c.google != 'whatever'
        assert not hasattr(c, 'extra')
        c.extra = extra
        c.google = 'whatever'
    # todo hmm. not sure what should do about new properties??
    assert not hasattr(c, 'extra')
    assert c.google != 'whatever'