Better error messages

src/fk_graph/cli.py

@@ -1,5 +1,5 @@
-from argparse import ArgumentParser
-from json import loads
+from argparse import ArgumentParser, ArgumentTypeError
+from json import JSONDecodeError, loads
 
 from sqlalchemy import create_engine
 
@@ -47,4 +47,13 @@ def _parse_args():
     return args
 
 def _to_list(json_input):
-    return loads(json_input)
+    argument_type_error = ArgumentTypeError(
+            "The --only-tables argument should be a list-like JSON string."
+        )
+    try:
+        parsed = loads(json_input)
+    except JSONDecodeError:
+        raise argument_type_error
+    if not isinstance(parsed, list):
+        raise argument_type_error
+    return parsed

tests/test_cli.py

@@ -79,3 +79,43 @@ def test_can_pass_in_list_of_tables_to_include(self):
         # Because of the javascripty-nature of the app, we can
         # only really inspect the status code.
         self.assertEqual(response.status_code, 200)
+
+    def test_errors_if_only_tables_is_not_json(self):
+        command = (
+            "fk-graph"
+            " --demo"
+            " --table=table_a"
+            " --primary-key=1"
+            " --only-tables='not json'"
+        )
+        completed_process = run(
+            shlex_split(command),
+            capture_output=True,
+            text=True,
+            timeout=5
+        )
+        self.assertEqual(completed_process.returncode, 2)
+        self.assertIn(
+            "The --only-tables argument should be a list-like JSON string.",
+            completed_process.stderr
+        )
+
+    def test_errors_if_only_tables_is_not_listlike_json(self):
+        command = (
+            "fk-graph"
+            " --demo"
+            " --table=table_a"
+            " --primary-key=1"
+            " --only-tables='{\"not\": \"a list\"}'"
+        )
+        completed_process = run(
+            shlex_split(command),
+            capture_output=True,
+            text=True,
+            timeout=5
+        )
+        self.assertEqual(completed_process.returncode, 2)
+        self.assertIn(
+            "The --only-tables argument should be a list-like JSON string.",
+            completed_process.stderr
+        )