fix: issue a fatal error when reading an empty mapping

Letting an empty mapping get loaded will corrupt langs.json.gz, so
immediately alert the user they cannot do that!

Also, don't override the rules_path if it's already initialized.

Fixes: 322

g2p/mappings/__init__.py

@@ -347,7 +347,10 @@ def export_to_dict(self, mapping_type="json", config_only=False):
                 exclude_none=True,
                 exclude={"parent_dir": True},
             )
-        model_dict["rules_path"] = f"{self.in_lang}_to_{self.out_lang}.{mapping_type}"
+        if not model_dict.get("rules_path"):
+            model_dict[
+                "rules_path"
+            ] = f"{self.in_lang}_to_{self.out_lang}.{mapping_type}"
         return model_dict
 
     def config_to_file(

g2p/mappings/utils.py

@@ -389,6 +389,10 @@ def load_from_file(path: Union[Path, str]) -> list:
         raise exceptions.IncorrectFileType(
             f"File {path} is not a valid mapping filetype."
         )
+    if not mapping:
+        raise exceptions.MalformedMapping(
+            f"Sorry, the file {path} does not contain any rules."
+        )
     return mapping
 
 

g2p/tests/public/mappings/case-feed/empty.csv

@@ -0,0 +1 @@
+t,t,,Actually empty is illegal so create at least one dummy rule

g2p/tests/test_z_local_config.py

@@ -22,6 +22,7 @@
 
 import yaml
 
+from g2p import exceptions
 from g2p.app import APP
 from g2p.cli import convert, generate_mapping
 from g2p.mappings import Mapping
@@ -131,14 +132,14 @@ def test_missing_files(self):
 
         with tempfile.TemporaryDirectory() as tmpdir:
             config_file = os.path.join(tmpdir, "abbrev-file-not-found.yaml")
-            with open(os.path.join(tmpdir, "empty.csv"), "wt", encoding="utf8") as f:
-                pass
+            with open(os.path.join(tmpdir, "tiny.csv"), "wt", encoding="utf8") as f:
+                f.write("a,b\n")
             with open(config_file, "wt", encoding="utf8") as f:
                 yaml.dump(
                     {
                         "mappings": [
                             {
-                                "rules_path": "empty.csv",
+                                "rules_path": "tiny.csv",
                                 "abbreviations_path": "no-such-file.csv",
                             }
                         ]
@@ -154,6 +155,26 @@ def test_missing_files(self):
                 results.output + str(results.exception),
             )
 
+    def test_empty_rules(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            config_file = os.path.join(tmpdir, "empty-rules-file.yaml")
+            with open(os.path.join(tmpdir, "empty.csv"), "wt", encoding="utf8") as f:
+                pass
+            with open(config_file, "wt", encoding="utf8") as f:
+                yaml.dump({"mappings": [{"rules_path": "empty.csv"}]}, f)
+            with self.assertRaises(exceptions.MalformedMapping) as e:
+                # This is a deep pydantic exception, we should raise MalformedMapping
+                Mapping.load_mapping_from_path(config_file)
+            self.assertIn("empty.csv does not contain any rules", str(e.exception))
+            results = self.runner.invoke(
+                convert, ["--config", config_file, "a", "b", "c"]
+            )
+            self.assertNotEqual(results.exit_code, 0)
+            self.assertIn(
+                "empty.csv does not contain any rules",
+                results.output + str(results.exception),
+            )
+
     def test_generate_mapping(self):
         """Use a local config to test generate mapping with --from and --to"""
         # This test is rather hacky, because it relies on the fact that calling