run_bot: Improve the error when a bot module can't be found.

zulip_bots/zulip_bots/run.py

@@ -4,6 +4,7 @@
 import logging
 import os
 import sys
+from pathlib import Path
 from typing import Optional
 
 from zulip_bots import finder
@@ -111,6 +112,28 @@ def exit_gracefully_if_bot_config_file_does_not_exist(bot_config_file: Optional[
         sys.exit(1)
 
 
+def print_bot_not_found_error(bot: str, registry: bool) -> None:
+    if registry:
+        print(
+            f'ERROR: Could not find a bot named "{bot}" in the zulip_bots registry.\n'
+            "Make sure the package that provides it is installed. Exiting now."
+        )
+        return
+
+    bundled_bot_path = Path(finder.current_dir, "bots", bot, bot + ".py")
+    print(
+        f"""ERROR: Could not load bot module for "{bot}". Looked for:
+  - a file at {os.path.abspath(bot)}
+  - {bundled_bot_path} (one of the bots included with zulip_bots)
+  - a Python module named "{bot}" that can be imported from PYTHONPATH
+
+If you wrote this bot yourself, try passing the path to its .py file, e.g.:
+    zulip-run-bot path/to/{Path(bot).stem}.py --config-file ~/zuliprc
+
+Exiting now."""
+    )
+
+
 def main() -> None:
     args = parse_args()
 
@@ -156,11 +179,11 @@ def main() -> None:
                 bot_name = lib_module.__name__
                 bot_source = "named module"
                 if args.provision:
-                    print("ERROR: Could not load bot's module for '{}'. Exiting now.")
+                    print(f"ERROR: Could not load bot's module for '{args.bot}'. Exiting now.")
                     sys.exit(1)
 
     if lib_module is None:
-        print("ERROR: Could not load bot module. Exiting now.")
+        print_bot_not_found_error(args.bot, args.registry)
         sys.exit(1)
 
     if not args.quiet:

zulip_bots/zulip_bots/tests/test_run.py

@@ -1,5 +1,7 @@
+import io
 import os
 import sys
+from contextlib import redirect_stdout
 from pathlib import Path
 from typing import Optional
 from unittest import TestCase, mock
@@ -127,6 +129,50 @@ def test_run_bot_by_module_name(self, mock_os_path_isfile: mock.Mock) -> None:
                         zulip_bots.run.main()
                         mock_import_module.assert_called_once_with(bot_module_name)
 
+    @patch("os.path.isfile", return_value=False)
+    def test_provision_with_bot_module_name_shows_bot_name(
+        self, mock_os_path_isfile: mock.Mock
+    ) -> None:
+        mock_bot_module = mock.Mock()
+        mock_bot_module.__name__ = "bot.module.name"
+        output = io.StringIO()
+        with patch("sys.argv", ["zulip-run-bot", "bot.module.name", "--provision"]):
+            with patch("importlib.import_module", return_value=mock_bot_module):
+                with redirect_stdout(output), self.assertRaises(SystemExit):
+                    zulip_bots.run.main()
+
+        self.assertIn("'bot.module.name'", output.getvalue())
+        self.assertNotIn("{}", output.getvalue())
+
+    @patch("os.path.isfile", return_value=False)
+    def test_bot_not_found_error_suggests_passing_path(
+        self, mock_os_path_isfile: mock.Mock
+    ) -> None:
+        output = io.StringIO()
+        with patch("sys.argv", ["zulip-run-bot", "mybot", "--config-file", "/path/to/config"]):
+            with patch("zulip_bots.finder.import_module_by_name", return_value=None):
+                with redirect_stdout(output), self.assertRaises(SystemExit):
+                    zulip_bots.run.main()
+
+        error = output.getvalue()
+        self.assertIn('Could not load bot module for "mybot"', error)
+        self.assertIn(str(Path("bots", "mybot", "mybot.py")), error)
+        self.assertIn("PYTHONPATH", error)
+        self.assertIn("zulip-run-bot path/to/mybot.py", error)
+
+    def test_bot_not_found_in_registry_error(self) -> None:
+        output = io.StringIO()
+        with patch(
+            "sys.argv", ["zulip-run-bot", "mybot", "--config-file", "/path/to/config", "-r"]
+        ):
+            with patch("zulip_bots.finder.metadata.entry_points", return_value=()):
+                with redirect_stdout(output), self.assertRaises(SystemExit):
+                    zulip_bots.run.main()
+
+        self.assertIn(
+            'Could not find a bot named "mybot" in the zulip_bots registry', output.getvalue()
+        )
+
 
 class TestBotLib(TestCase):
     def test_extract_query_without_mention(self) -> None: