@hundreds-father-404 is there a reason

SpecsParser.parse_spec

has return type

Union[AddressSpec, FilesystemSpec]

instead of just

Spec

?

Yep, because then when we do

isinstance(AddressSpec)

, MyPy knows if that's not true it would be a

FilesystemSpec

so mypy in general doesn't assume it knows about all subclasses of a class that might exist?

exactly, because it cannot do that. So we would need to do:

if isinstance(AddressSpec):
  ...
elif isinstance(FilesystemSpec):
  ...
else:
  raise Unexpected