Revert the default type of nitpick_ignore[_regex] to list

CHANGES

@@ -16,6 +16,9 @@ Features added
 Bugs fixed
 ----------
 
+* #11355: Revert the default type of :confval:`nitpick_ignore` and
+  :confval:`nitpick_ignore_regex` to ``list``.
+
 Testing
 --------
 

sphinx/config.py

@@ -132,8 +132,8 @@ class Config:
         'needs_extensions': ({}, None, []),
         'manpages_url': (None, 'env', []),
         'nitpicky': (False, None, []),
-        'nitpick_ignore': (set(), None, [set, list, tuple]),
-        'nitpick_ignore_regex': (set(), None, [set, list, tuple]),
+        'nitpick_ignore': ([], None, [set, list, tuple]),
+        'nitpick_ignore_regex': ([], None, [set, list, tuple]),
         'numfig': (False, 'env', []),
         'numfig_secnum_depth': (1, 'env', []),
         'numfig_format': ({}, 'env', []),  # will be initialized in init_numfig_format()

tests/test_config.py

@@ -427,3 +427,18 @@ def test_conf_py_no_language(tempdir):
 
     # Then the language is coerced to English
     assert cfg.language == "en"
+
+
+def test_conf_py_nitpick_ignore_list(tempdir):
+    """Regression test for #11355."""
+
+    # Given a conf.py file with no language attribute
+    (tempdir / 'conf.py').write_text("", encoding='utf-8')
+
+    # When we load conf.py into a Config object
+    cfg = Config.read(tempdir, {}, None)
+    cfg.init_values()
+
+    # Then the default nitpick_ignore[_regex] is an empty list
+    assert cfg.nitpick_ignore == []
+    assert cfg.nitpick_ignore_regex == []