gh-152142: Fix inverted DOMBuilderFilter result for notation declarations

Lib/test/test_minidom.py

@@ -1784,5 +1784,38 @@ def test_cdata_parsing(self):
         dom2 = parseString(dom1.toprettyxml())
         self.checkWholeText(dom2.getElementsByTagName('node')[0].firstChild, '</data>')
 
+    def test_notation_decl_filter(self):
+        # gh-152142: a DOMBuilderFilter accepting a notation must keep it and
+        # rejecting it must drop it, matching entity declarations.
+        from xml.dom.expatbuilder import makeBuilder
+        from xml.dom.xmlbuilder import DOMBuilderFilter, Options
+
+        source = (b'<?xml version="1.0"?>\n'
+                  b'<!DOCTYPE root [\n'
+                  b'  <!NOTATION gif PUBLIC "image/gif">\n'
+                  b'  <!ENTITY logo SYSTEM "logo.gif" NDATA gif>\n'
+                  b']>\n'
+                  b'<root/>')
+
+        class Filter(DOMBuilderFilter):
+            def __init__(self, result):
+                self.result = result
+            def acceptNode(self, node):
+                if node.nodeType == Node.NOTATION_NODE:
+                    return self.result
+                return self.FILTER_ACCEPT
+
+        def counts(filt):
+            options = Options()
+            options.filter = filt
+            doctype = makeBuilder(options).parseFile(io.BytesIO(source)).doctype
+            return doctype.notations.length, doctype.entities.length
+
+        # The entity must stay untouched in every case; only the notation
+        # follows the filter result.
+        self.assertEqual(counts(None), (1, 1))
+        self.assertEqual(counts(Filter(DOMBuilderFilter.FILTER_ACCEPT)), (1, 1))
+        self.assertEqual(counts(Filter(DOMBuilderFilter.FILTER_REJECT)), (0, 1))
+
 if __name__ == "__main__":
     unittest.main()

Lib/xml/dom/expatbuilder.py

@@ -320,7 +320,7 @@ def entity_decl_handler(self, entityName, is_parameter_entity, value,
     def notation_decl_handler(self, notationName, base, systemId, publicId):
         node = self.document._create_notation(notationName, publicId, systemId)
         self.document.doctype.notations._seq.append(node)
-        if self._filter and self._filter.acceptNode(node) == FILTER_ACCEPT:
+        if self._filter and self._filter.acceptNode(node) == FILTER_REJECT:
             del self.document.doctype.notations._seq[-1]
 
     def comment_handler(self, data):

Misc/NEWS.d/next/Library/2026-06-25-09-43-51.gh-issue-152142.cabd13.rst

@@ -0,0 +1,4 @@
+Fix the handling of notation declarations by
+:class:`!xml.dom.xmlbuilder.DOMBuilderFilter` in :mod:`xml.dom.minidom`. A
+notation that the filter accepted was dropped and a rejected one was kept, the
+opposite of the filter contract and of entity declarations.