gh-116957: configparser: Do post-process values after DuplicateOptionError (GH-116958)

If you catch DuplicateOptionError / DuplicateSectionError when reading a
config file (the intention is to skip invalid config files) and then
attempt to use the ConfigParser instance, any values it *had* read
successfully so far, were stored as a list instead of string! Later
`get` calls would raise "AttributeError: 'list' object has no attribute
'find'" from somewhere deep in the interpolation code.

Lib/configparser.py

@@ -961,6 +961,7 @@ class RawConfigParser(MutableMapping):
         lineno = 0
         indent_level = 0
         e = None                              # None, or an exception
+        try:
             for lineno, line in enumerate(fp, start=1):
                 comment_start = sys.maxsize
                 # strip inline comments
@@ -1056,6 +1057,7 @@ class RawConfigParser(MutableMapping):
                             # raised at the end of the file and will contain a
                             # list of all bogus lines
                             e = self._handle_error(e, fpname, lineno, line)
+        finally:
             self._join_multiline_values()
         # if any parsing errors occurred, raise an exception
         if e:

Lib/test/test_configparser.py

@@ -646,6 +646,21 @@ boolean {0[0]} NO
                                      "'opt' in section 'Bar' already exists")
             self.assertEqual(e.args, ("Bar", "opt", "<dict>", None))
 
+    def test_get_after_duplicate_option_error(self):
+        cf = self.newconfig()
+        ini = textwrap.dedent("""\
+            [Foo]
+            x{equals}1
+            y{equals}2
+            y{equals}3
+        """.format(equals=self.delimiters[0]))
+        if self.strict:
+            with self.assertRaises(configparser.DuplicateOptionError):
+                cf.read_string(ini)
+        else:
+            cf.read_string(ini)
+        self.assertEqual(cf.get('Foo', 'x'), '1')
+
     def test_write(self):
         config_string = (
             "[Long Line]\n"

Misc/NEWS.d/next/Library/2024-03-18-14-36-50.gh-issue-116957.dTCs4f.rst

@@ -0,0 +1,3 @@
+configparser: Don't leave ConfigParser values in an invalid state (stored as
+a list instead of a str) after an earlier read raised DuplicateSectionError
+or DuplicateOptionError.