bpo-46607: Add DeprecationWarning to LegacyInterpolation

Doc/whatsnew/3.11.rst

@@ -486,6 +486,12 @@ Deprecated
 
   (Contributed by Hugo van Kemenade in :issue:`45173`.)
 
+* :class:`configparser.LegacyInterpolation` has been deprecated in the docstring
+  since Python 3.2. It now emits a :exc:`DeprecationWarning` and will be removed
+  in Python 3.13. Use :class:`configparser.BasicInterpolation` or
+  :class:`configparser.ExtendedInterpolation instead.
+  (Contributed by Hugo van Kemenade in :issue:`46607`.)
+
 * The :func:`locale.getdefaultlocale` function is deprecated and will be
   removed in Python 3.13. Use :func:`locale.setlocale`,
   :func:`locale.getpreferredencoding(False) <locale.getpreferredencoding>` and

Lib/configparser.py

@@ -525,6 +525,15 @@ class LegacyInterpolation(Interpolation):
 
     _KEYCRE = re.compile(r"%\(([^)]*)\)s|.")
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        warnings.warn(
+            "LegacyInterpolation has been deprecated since Python 3.2 "
+            "and will be removed from the configparser module in Python 3.13. "
+            "Use BasicInterpolation or ExtendedInterpolation instead.",
+            DeprecationWarning, stacklevel=2
+        )
+
     def before_get(self, parser, section, option, value, vars):
         rawval = value
         depth = MAX_INTERPOLATION_DEPTH

Lib/test/test_configparser.py

@@ -1666,6 +1666,14 @@ def test_safeconfigparser_deprecation(self):
         for warning in w:
             self.assertTrue(warning.category is DeprecationWarning)
 
+    def test_legacyinterpolation_deprecation(self):
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always", DeprecationWarning)
+            configparser.LegacyInterpolation()
+        self.assertGreaterEqual(len(w), 1)
+        for warning in w:
+            self.assertIs(warning.category, DeprecationWarning)
+
     def test_sectionproxy_repr(self):
         parser = configparser.ConfigParser()
         parser.read_string("""

Misc/NEWS.d/next/Library/2022-01-26-18-30-34.bpo-46607.xnhT4a.rst

@@ -0,0 +1,3 @@
+Add :exc:`DeprecationWarning` to :class:`LegacyInterpolation`, deprecated in
+the docstring since Python 3.2. Will be removed in Python 3.13. Use
+:class:`BasicInterpolation` or :class:`ExtendedInterpolation` instead.