Resource is an abstract class, be clear about it

the consequence of this is that we need to declare the non-existent
function destroy in `Resource`.
As a consequence, we have to explicitly declare it in the slightly
specialized classes that directly inherit from it, `DefaultResource`
and `ListableResource`. We "implement" it by raising an error.

This patch fixes #156, and has been once again discovered by `mypy
--strict` (#131).

Author: igalic

iocage/lib/Resource.py

@@ -23,6 +23,7 @@
 # POSSIBILITY OF SUCH DAMAGE.
 import typing
 import os.path
+import abc
 
 import libzfs
 
@@ -41,7 +42,7 @@
 import iocage.lib.Config.File  # noqa: F401
 
 
-class Resource:
+class Resource(metaclass=abc.ABCMeta):
     """
     iocage resource
 
@@ -124,6 +125,10 @@ def __init__(
         elif dataset is not None:
             self.dataset = dataset
 
+    @abc.abstractmethod
+    def destroy(self):
+        pass
+
     @property
     def pool(self) -> libzfs.ZFSPool:
         return self.zfs.get_pool(self.dataset_name)
@@ -321,6 +326,9 @@ def __init__(
             logger=logger
         )
 
+    def destroy(self):
+        raise NotImplementedError("destroy unimplemented for DefaultResource")
+
     def save(self) -> None:
         self.write_config(self.config.user_data)
 
@@ -349,6 +357,9 @@ def __init__(
 
         self.filters = filters
 
+    def destroy(self):
+        raise NotImplementedError("destroy unimplemented for ListableResource")
+
     def __iter__(
         self
     ) -> typing.Generator['iocage.lib.Resource.Resource', None, None]: