gh-157456: Fix configparser parsing an empty value with a whitespace-ending delimiter

An option whose delimiter ends in whitespace (e.g. ``delimiters=(' ',)``) and
has an empty value could not be parsed, and the parser could not read back the
output produced by ``write()``: ``_read`` matched option lines against
``line.clean``, which is ``str.strip``-ed on both sides, so for a whitespace
delimiter the trailing whitespace that separates the key from the (empty) value
was removed and ``OPTCRE`` no longer matched.

Match the option regex against a form that preserves trailing whitespace. The
matched value is stripped immediately afterwards, so ordinary values keep no
trailing whitespace, and the other uses of ``line.clean`` are unchanged.

Add a regression test; the gh-156353 whitespace-delimiter tests only used
non-empty values, so this edge was unpinned.

Author: winklemad

Lib/configparser.py

@@ -599,8 +599,9 @@ def __init__(self, full_prefixes, inline_prefixes):
         )
         self.pattern = re.compile('|'.join(itertools.chain(full_patterns, inline_patterns)))
 
-    def strip(self, text):
-        return self.pattern.sub('', text).rstrip()
+    def strip(self, text, *, rstrip=True):
+        text = self.pattern.sub('', text)
+        return text.rstrip() if rstrip else text
 
     def wrap(self, text):
         return _Line(text, self)
@@ -1156,7 +1157,13 @@ def _handle_option(self, st, line, fpname):
         # an option line?
         st.indent_level = st.cur_indent_level
 
-        mo = self._optcre.match(line.clean)
+        # `line.clean` strips trailing whitespace, but for a whitespace delimiter
+        # that whitespace is the separator before an (empty) value, so match
+        # against a form that preserves it.  `optval` is stripped below, so
+        # ordinary values are unaffected.
+        match_target = self._comments.strip(str(line).strip('\r\n').lstrip(),
+                                             rstrip=False)
+        mo = self._optcre.match(match_target)
         if not mo:
             # a non-fatal parsing error occurred. set up the
             # exception but keep going. the exception will be

Lib/test/test_configparser.py

@@ -385,6 +385,23 @@ def test_any_delimiter(self, delimiter, space_before, space_after):
         self.assertEqual(cf.options('all'), ['foo'])
         self.assertEqual(cf.get('all', 'foo'), 'bar=baz')
 
+    def test_whitespace_delimiter_empty_value(self):
+        # gh-157456: an option with a whitespace-ending delimiter and an empty
+        # value must parse rather than raise ParsingError, and the parser must
+        # read back the output produced by write().
+        cf = self.newconfig(delimiters=(' ',))
+        cf.read_string("[all]\nkey \n")
+        # With allow_no_value a whitespace delimiter cannot distinguish an
+        # empty value from a valueless option, so the value is None there;
+        # otherwise it is the empty string.
+        expected = None if cf._allow_no_value else ''
+        self.assertEqual(cf.get('all', 'key'), expected)
+        output = io.StringIO()
+        cf.write(output)
+        cf2 = self.newconfig(delimiters=(' ',))
+        cf2.read_string(output.getvalue())
+        self.assertEqual(cf2.get('all', 'key'), expected)
+
     def test_basic_from_dict(self):
         config = {
             "Foo Bar": {

Misc/NEWS.d/next/Library/2026-09-14-01-10-00.gh-issue-157456.p3Rw8k.rst

@@ -0,0 +1,4 @@
+Fix :mod:`configparser` failing to parse an option with an empty value when
+the delimiter ends in whitespace (for example ``delimiters=(' ',)``). Such an
+option, including the output produced by :meth:`~configparser.RawConfigParser.write`,
+is now read back correctly.