gh-158225: Fix wsgiref Headers __setitem__ validation order

Author: Tony123-tech

Lib/test/test_wsgiref.py

@@ -592,6 +592,14 @@ def testMappingInterface(self):
         self.assertEqual(h["foo"],"baz")
         self.assertEqual(h["zoo"],"whee")
 
+    def testSetItemValidationFailureKeepsOldValue(self):
+        # gh-158225: __setitem__ must not delete the old header
+        # before validating the new name and value.
+        h = Headers([("Content-Type", "text/html")])
+        with self.assertRaises(ValueError):
+            h["Content-Type"] = "bad\x00value"
+        self.assertEqual(h["Content-Type"], "text/html")
+
     def testRequireList(self):
         self.assertRaises(TypeError, Headers, "foo")
 

Lib/wsgiref/headers.py

@@ -59,10 +59,11 @@ def __len__(self):
 
     def __setitem__(self, name, val):
         """Set the value of a header."""
+        name = self._convert_string_type(name, name=True)
+        val = self._convert_string_type(val, name=False)
         del self[name]
-        self._headers.append(
-            (self._convert_string_type(name, name=True), self._convert_string_type(val, name=False)))
-
+        self._headers.append((name, val))
+         
     def __delitem__(self,name):
         """Delete all occurrences of a header, if present.
 

Misc/NEWS.d/next/Library/2026-09-26-17-24-55.gh-issue-158225.AbCdEf.rst

@@ -0,0 +1,2 @@
+Fix :meth:`wsgiref.headers.Headers.__setitem__` to keep the existing
+header value when validation of the new value fails. Patch by Tony Leung.