Raise a useful error if `theme.conf` doesn't exist (#11671)

CHANGES.rst

@@ -16,6 +16,9 @@ Features added
 Bugs fixed
 ----------
 
+* #11668: Raise a useful error when ``theme.conf`` is missing.
+  Patch by Vinay Sajip.
+
 Testing
 -------
 

sphinx/theming.py

@@ -69,7 +69,10 @@ class Theme:
             extract_zip(theme_path, self.themedir)
 
         self.config = configparser.RawConfigParser()
-        self.config.read(path.join(self.themedir, THEMECONF), encoding='utf-8')
+        config_file_path = path.join(self.themedir, THEMECONF)
+        if not os.path.isfile(config_file_path):
+            raise ThemeError(__('theme configuration file %r not found') % config_file_path)
+        self.config.read(config_file_path, encoding='utf-8')
 
         try:
             inherit = self.config.get('theme', 'inherit')

tests/test_theming.py

@@ -6,7 +6,7 @@ import alabaster
 import pytest
 
 import sphinx.builders.html
-from sphinx.theming import ThemeError
+from sphinx.theming import Theme, ThemeError
 
 
 @pytest.mark.sphinx(
@@ -62,6 +62,13 @@ def test_theme_api(app, status, warning):
     assert not os.path.exists(themedir)
 
 
+def test_nonexistent_theme_conf(tmp_path):
+    # Check that error occurs with a non-existent theme.conf
+    # (https://github.com/sphinx-doc/sphinx/issues/11668)
+    with pytest.raises(ThemeError):
+        Theme('dummy', str(tmp_path), None)
+
+
 @pytest.mark.sphinx(testroot='double-inheriting-theme')
 def test_double_inheriting_theme(app, status, warning):
     assert app.builder.theme.name == 'base_theme2'