gh-70999: Preserve caller vars in ExtendedInterpolation nested interpolation

ExtendedInterpolation honoured caller-supplied ``vars`` only at the first
level of interpolation. When a value referenced another option in the same
section, the recursive call rebuilt the lookup map from
``parser.items(section, raw=True)``, discarding the map that carried ``vars``.

Mirror BasicInterpolation, which threads the same ``(section, map)`` through
recursion, by reusing ``map`` when the reference stays in the current section
and only rebuilding it when crossing into another section (so ``vars`` do not
leak across sections).

Author: nikolauspschuetz

Lib/configparser.py

@@ -547,9 +547,12 @@ def _interpolate_some(self, parser, option, accum, rest, section, map,
                 if v is None:
                     continue
                 if "$" in v:
+                    if sect == section:
+                        submap = map
+                    else:
+                        submap = dict(parser.items(sect, raw=True))
                     self._interpolate_some(parser, opt, accum, v, sect,
-                                           dict(parser.items(sect, raw=True)),
-                                           depth + 1)
+                                           submap, depth + 1)
                 else:
                     accum.append(v)
             else:

Lib/test/test_configparser.py

@@ -1334,6 +1334,30 @@ def test_other_errors(self):
         with self.assertRaises(ValueError):
             cf['interpolation fail']['case6'] = "BLACK $ABBATH"
 
+    def test_get_with_vars_nested(self):
+        # gh-70999: caller-supplied ``vars`` must be honoured at every level
+        # of a same-section interpolation chain, not just the first.
+        cf = self.fromstring(textwrap.dedent("""
+            [section]
+            a = ${b}
+            b = ${c}
+            c = default
+
+            [cross]
+            via = ${section:c}
+        """).strip())
+
+        eq = self.assertEqual
+        # Directly referencing the overridden option already worked.
+        eq(cf.get('section', 'b', vars={'c': 'OVERRIDE'}), 'OVERRIDE')
+        # Reaching it through another same-section option must too.
+        eq(cf.get('section', 'a', vars={'c': 'OVERRIDE'}), 'OVERRIDE')
+        # Without an override the configured value is still used.
+        eq(cf.get('section', 'a'), 'default')
+        # ``vars`` are scoped to the requested section and must not leak
+        # into a different section reached via ``${section:option}``.
+        eq(cf.get('cross', 'via', vars={'c': 'OVERRIDE'}), 'default')
+
 
 class ConfigParserTestCaseNoValue(ConfigParserTestCase):
     allow_no_value = True

Misc/NEWS.d/next/Library/2026-08-15-12-00-00.gh-issue-70999.NZ96LZ.rst

@@ -0,0 +1,3 @@
+Fix :class:`configparser.ExtendedInterpolation` dropping caller-supplied
+``vars`` during nested interpolation within a section.  Patch by Nikolaus
+Schuetz.