gh-157466: Fix configparser writing a trailing space for empty values

Lib/configparser.py

@@ -997,6 +997,13 @@ def _write_section(self, fp, section_name, section_items, delimiter, unnamed=Fal
                 # Convert all possible line-endings into '\n\t'
                 value = (delimiter + str(value).replace('\r\n', '\n')
                          .replace('\r', '\n').replace('\n', '\n\t'))
+                if value == delimiter and not self._delimiters[0][-1:].isspace():
+                    # The value is empty, so the space that
+                    # `space_around_delimiters` appends would be left
+                    # dangling at the end of the line.  Keep it only when
+                    # the delimiter itself ends in whitespace, where it is
+                    # needed to read the option back.
+                    value = value.rstrip(' ')
             else:
                 value = ""
             fp.write("{}{}\n".format(key, value))

Lib/test/test_configparser.py

@@ -752,6 +752,27 @@ def test_write(self):
                     )
             self.assertEqual(output.getvalue(), expect_string)
 
+    def test_write_empty_value(self):
+        # gh-157466: an empty value must not leave the space that
+        # `space_around_delimiters` appends dangling at end of line,
+        # while a value that really ends in whitespace keeps it.
+        cf = self.newconfig()
+        cf.add_section('sect')
+        cf.set('sect', 'empty', '')
+        cf.set('sect', 'padded', 'value  ')
+        for space_around_delimiters in (True, False):
+            delimiter = self.delimiters[0]
+            if space_around_delimiters:
+                delimiter = " {} ".format(delimiter)
+            output = io.StringIO()
+            cf.write(output, space_around_delimiters=space_around_delimiters)
+            self.assertEqual(
+                output.getvalue(),
+                "[sect]\n"
+                "empty{}\n"
+                "padded{}value  \n"
+                "\n".format(delimiter.rstrip(' '), delimiter))
+
     def test_set_string_types(self):
         cf = self.fromstring("[sect]\n"
                              "option1{eq}foo\n".format(eq=self.delimiters[0]))

Misc/NEWS.d/next/Library/2026-09-14-11-30-00.gh-issue-157466.gkzXKm.rst

@@ -0,0 +1,5 @@
+Fix :mod:`configparser` leaving a trailing space at the end of the line when
+:meth:`~configparser.ConfigParser.write` writes an option with an empty
+value and ``space_around_delimiters`` is true. The space is still written
+when the delimiter itself ends in whitespace, where it is needed to read the
+option back.