save relative paths to config file, if possible

Although using paths that are *not* relative to the current working directory (cwd) is discouraged,
this *should* still be possible. So, if the path is not relative to cwd, we just save the absolute path.

src/tufup/repo/__init__.py

@@ -567,15 +567,30 @@ def app_version(self) -> str:
 
     @classmethod
     def get_config_file_path(cls) -> pathlib.Path:
+        # config must be stored in current working directory
         return pathlib.Path.cwd() / cls.config_filename
 
     def save_config(self):
         """Save current configuration."""
-        # todo: write directories relative to config file dir?
-        file_path = self.get_config_file_path()
-        file_path.write_text(
+        config_file_path = self.get_config_file_path()
+        # make paths relative to current working directory (cwd),
+        # if possible, otherwise keep absolute paths (note, to avoid
+        # confusion, using paths other than cwd is discouraged)
+        temp_config_dict = self.config_dict  # note self.config_dict is a property
+        for key in ['repo_dir', 'keys_dir']:
+            try:
+                temp_config_dict[key] = temp_config_dict[key].relative_to(
+                    pathlib.Path.cwd()
+                )
+            except ValueError:
+                logger.warning(
+                    f'Saving *absolute* path in config, because the path is'
+                    f' not relative to cwd: {temp_config_dict[key]}'
+                )
+        # write file
+        config_file_path.write_text(
             data=json.dumps(
-                self.config_dict, default=str, sort_keys=True, indent=4
+                temp_config_dict, default=str, sort_keys=True, indent=4
             ),
             encoding='utf-8',
         )

tests/test_repo.py

@@ -529,8 +529,8 @@ def test_save_config(self):
         self.assertTrue(repo.get_config_file_path().exists())
         config_file_text = repo.get_config_file_path().read_text()
         print(config_file_text)  # for convenience
-        # paths saved to config file must be relative (note that we should
-        # still be able to *read* absolute paths from the config file)
+        # paths saved to config file are relative to current working
+        # directory (cwd) if possible (otherwise absolute paths are saved)
         config_dict = json.loads(config_file_text)
         for key in ['repo_dir', 'keys_dir']:
             with self.subTest(msg=key):