Provide a friendly message if version option is required (#507)

src/towncrier/build.py

@@ -15,7 +15,7 @@
 
 import click
 
-from click import Context, Option
+from click import Context, Option, UsageError
 
 from towncrier import _git
 
@@ -151,6 +151,18 @@ def __main(
     base_directory, config = load_config_from_options(directory, config_file)
     to_err = draft
 
+    if project_version is None:
+        project_version = config.version
+    if project_version is None:
+        if not config.package:
+            raise UsageError(
+                "'--version' is required since the config file does "
+                "not contain 'version' or 'package'."
+            )
+        project_version = get_version(
+            os.path.join(base_directory, config.package_dir), config.package
+        ).strip()
+
     click.echo("Loading template...", err=to_err)
     if isinstance(config.template, tuple):
         template = resources.read_text(*config.template)
@@ -182,13 +194,6 @@ def __main(
         fragment_contents, config.types, all_bullets=config.all_bullets
     )
 
-    if project_version is None:
-        project_version = config.version
-        if project_version is None:
-            project_version = get_version(
-                os.path.join(base_directory, config.package_dir), config.package
-            ).strip()
-
     if project_name is None:
         project_name = config.name
         if not project_name:

src/towncrier/newsfragments/507.misc

@@ -0,0 +1 @@
+A friendly message is now provided, when it's necessary to pass the ``--version`` option explicitly.

src/towncrier/test/test_build.py

@@ -511,6 +511,19 @@ def test_needs_config(self):
         self.assertEqual(1, result.exit_code, result.output)
         self.assertTrue(result.output.startswith("No configuration file found."))
 
+    @with_isolated_runner
+    def test_needs_version(self, runner: CliRunner):
+        """
+        If the configuration file doesn't specify a version or a package, the version
+        option is required.
+        """
+        write("towncrier.toml", "[tool.towncrier]")
+
+        result = runner.invoke(_main, ["--draft"], catch_exceptions=False)
+
+        self.assertEqual(2, result.exit_code)
+        self.assertIn("Error: '--version' is required", result.output)
+
     def test_projectless_changelog(self):
         """In which a directory containing news files is built into a changelog