diff --git a/Lib/test/test_xml_etree.py b/Lib/test/test_xml_etree.py
index fd4a38527fd..fcb1f7fdfbb 100644
--- a/Lib/test/test_xml_etree.py
+++ b/Lib/test/test_xml_etree.py
@@ -108,6 +108,19 @@ EXTERNAL_ENTITY_XML = """\
 <document>&entity;</document>
 """
 
+ATTLIST_XML = """\
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE Foo [
+<!ELEMENT foo (bar*)>
+<!ELEMENT bar (#PCDATA)*>
+<!ATTLIST bar xml:lang CDATA "eng">
+<!ENTITY qux "quux">
+]>
+<foo>
+<bar>&qux;</bar>
+</foo>
+"""
+
 def checkwarnings(*filters, quiet=False):
     def decorator(test):
         def newtest(*args, **kwargs):
@@ -1354,6 +1367,12 @@ class ElementTreeTest(unittest.TestCase):
         self.assertEqual(serialize(root, method='html'),
                 '<cirriculum status="public" company="example"></cirriculum>')
 
+    def test_attlist_default(self):
+        # Test default attribute values; See BPO 42151.
+        root = ET.fromstring(ATTLIST_XML)
+        self.assertEqual(root[0].attrib,
+                         {'{http://www.w3.org/XML/1998/namespace}lang': 'eng'})
+
 
 class XMLPullParserTest(unittest.TestCase):
 
diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index 7a269001d6e..168418e466c 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -1560,7 +1560,6 @@ class XMLParser:
         # Configure pyexpat: buffering, new-style attribute handling.
         parser.buffer_text = 1
         parser.ordered_attributes = 1
-        parser.specified_attributes = 1
         self._doctype = None
         self.entity = {}
         try:
@@ -1580,7 +1579,6 @@ class XMLParser:
         for event_name in events_to_report:
             if event_name == "start":
                 parser.ordered_attributes = 1
-                parser.specified_attributes = 1
                 def handler(tag, attrib_in, event=event_name, append=append,
                             start=self._start):
                     append((event, start(tag, attrib_in)))
diff --git a/Misc/NEWS.d/next/Library/2020-10-26-18-01-09.bpo-42151.et5f7s.rst b/Misc/NEWS.d/next/Library/2020-10-26-18-01-09.bpo-42151.et5f7s.rst
new file mode 100644
index 00000000000..6361f2c088d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-10-26-18-01-09.bpo-42151.et5f7s.rst
@@ -0,0 +1,3 @@
+Make the pure Python implementation of :mod:`xml.etree.ElementTree` behave
+the same as the C implementation (:mod:`_elementree`) regarding default
+attribute values (by not setting ``specified_attributes=1``).