gh-161317: configparser.write() raises InvalidWriteError for multiline values with leading whitespace

Lib/configparser.py

@@ -1087,9 +1087,15 @@ def _write_section(self, fp, section_name, section_items, delimiter, unnamed=Fal
             value = self._interpolation.before_write(self, section_name, key, value)
             if value is not None or not self._allow_no_value:
                 # Convert all possible line-endings into '\n\t'
-                value = delimiter + str(value).replace('\r\n', '\n').replace(
-                    '\r', '\n'
-                ).replace('\n', '\n\t')
+                str_value = str(value).replace('\r\n', '\n').replace('\r', '\n')
+                for continuation in str_value.split('\n')[1:]:
+                    if continuation and continuation[0] in ' \t':
+                        raise InvalidWriteError(
+                            f"Cannot write value for key {key!r}; "
+                            f"continuation line has leading whitespace "
+                            f"that would be silently stripped on read-back"
+                        )
+                value = delimiter + str_value.replace('\n', '\n\t')
             else:
                 value = ""
             fp.write(f"{key}{value}\n")

Lib/test/test_configparser.py

@@ -2498,6 +2498,14 @@ def test_section_bracket_in_key(self):
             cfg.write(output)
         output.close()
 
+    def test_leading_whitespace_in_multiline_value(self):
+        cfg = configparser.ConfigParser()
+        cfg.add_section('section1')
+        cfg.set('section1', 'key', 'line1\n\tline2')
+        output = io.StringIO()
+        with self.assertRaises(configparser.InvalidWriteError):
+            cfg.write(output)
+        output.close()
 
 class ReDoSTestCase(unittest.TestCase):
     """Regression tests for quadratic regex backtracking (gh-146333)."""

Misc/NEWS.d/next/Library/2026-11-04-18-30-35.gh-issue-161317.4XniSh.rst

@@ -0,0 +1,3 @@
+:mod:`configparser` now raises :exc:`~configparser.InvalidWriteError` when
+writing a value whose continuation lines contain leading whitespace, since
+that whitespace would be silently stripped on read-back.