are we going to recreate a runtime type checking system?

What do you mean? We use MyPy for everything, like indicating that

Compatibility.value

is

List[str]

but that

Compatibility.raw_value

is

Optional[Union[str, Iterable[str]]]

. The only point that is not safe is that we assume the type annotation for

raw_value

is correct. There’s nothing stopping a user from saying

compatibility=1.0

in a BUILD file. Our type hint that

raw_value: Optional[Union[str, Iterable[str]]]

is now a lie. So, possibly, we want to have very basic Python runtime type checks. For a boolean field, for example, nothing more complex than

if not isinstance(raw_value, bool): raise ValueError

checking for runtime types in a one-off manner without greater structure becomes super complex super easily

checking for runtime types in a one-off manner without greater structure becomes super complex super easily

There is a very nice structure thanks to @average-vr-56795’s suggestion. See https://github.com/pantsbuild/pants/pull/9296#discussion_r392434402 tl;dr: the vast majority of

Field

definitions won’t need to do anything more than declare their alias and possibly a default value, thanks to Field types we expose like

BoolField

,

StringField

, etc.

class ZipSafe(BoolField):
   alias: ClassVar = "zip_safe"
   default: ClassVar = True

That’s it 🎉