diff -r 1f1498fe50e5 Lib/test/test_xml_etree.py
--- a/Lib/test/test_xml_etree.py	Wed Dec 04 23:29:51 2013 +0100
+++ b/Lib/test/test_xml_etree.py	Thu Dec 05 00:30:46 2013 -0800
@@ -1612,6 +1612,24 @@
         ET.register_namespace('test10777', 'http://myuri/')
         ET.register_namespace('test10777', 'http://myuri/')
 
+    def test_issue17088(self):
+        # Unprefixed attributes are unqualified even if a default namespace is in
+        # effect.
+        e = ET.XML('<pf:name1 xmlns:pf="space1" xmlns:pf2="space2" attr="value">'
+                   '<pf:name2 attr="value2" pf2:attr="value3"/></pf:name1>')
+        self.assertEqual(e.tag, '{space1}name1')
+        self.assertEqual(e[0].attrib, { 'attr': 'value2', '{space2}attr': 'value3' })
+
+        pi = b"<?xml version='1.0' encoding='ascii'?>\n"
+        self.assertEqual(serialize(e),
+                         '<ns0:name1 xmlns:ns0="space1" xmlns:ns1="space2" attr="value">'
+                         '<ns0:name2 attr="value2" ns1:attr="value3" /></ns0:name1>')
+        self.assertEqual(serialize(e, default_namespace="space1"),
+                         '<name1 xmlns="space1" xmlns:ns1="space2" attr="value">'
+                         '<name2 attr="value2" ns1:attr="value3" /></name1>')
+        #self.assertEqual(serialize(e, default_namespace="space2"),
+        #                 '<ns1:name1 xmlns="space2" xmlns:ns1="space1" xmlns:ns2="space2" attr="value">'
+        #                 '<ns1:name2 attr="value2" ns2:attr="value3" /></ns1:name1>')
 
 # --------------------------------------------------------------------
 
diff -r 1f1498fe50e5 Lib/xml/etree/ElementTree.py
--- a/Lib/xml/etree/ElementTree.py	Wed Dec 04 23:29:51 2013 +0100
+++ b/Lib/xml/etree/ElementTree.py	Thu Dec 05 00:30:46 2013 -0800
@@ -847,7 +847,7 @@
     if default_namespace:
         namespaces[default_namespace] = ""
 
-    def add_qname(qname):
+    def add_qname(qname, is_attrname):
         # calculate serialized qname representation
         try:
             if qname[:1] == "{":
@@ -862,10 +862,24 @@
                 if prefix:
                     qnames[qname] = "%s:%s" % (prefix, tag)
                 else:
+                    # FIXME: (if is_attrname and prefix == ''): If an
+                    # attribute name is in the default namespace, we
+                    # will write it out without a prefix, which is
+                    # incorrect (an unprefixed attribute is always
+                    # unqualified, even if a default namespace is in
+                    # effect). For completely correct behavior, we
+                    # would need to emit both a default namespace
+                    # declaration *and* a prefix declaration for the
+                    # use of attributes.
                     qnames[qname] = tag # default element
             else:
-                if default_namespace:
-                    # FIXME: can this be handled in XML 1.0?
+                if default_namespace and not is_attrname:
+                    # A default namespace can be undeclared
+                    # (see http://www.w3.org/TR/REC-xml-names/#defaulting)
+                    # but only by placing an xmlns="" attribute on the
+                    # element and possibly re-declaring the default
+                    # namespace for child elements. Our serializers
+                    # can't do that. FIXME.
                     raise ValueError(
                         "cannot use non-qualified names with "
                         "default_namespace option"
@@ -879,22 +893,22 @@
         tag = elem.tag
         if isinstance(tag, QName):
             if tag.text not in qnames:
-                add_qname(tag.text)
+                add_qname(tag.text, False)
         elif isinstance(tag, str):
             if tag not in qnames:
-                add_qname(tag)
+                add_qname(tag, False)
         elif tag is not None and tag is not Comment and tag is not PI:
             _raise_serialization_error(tag)
         for key, value in elem.items():
             if isinstance(key, QName):
                 key = key.text
             if key not in qnames:
-                add_qname(key)
+                add_qname(key, True)
             if isinstance(value, QName) and value.text not in qnames:
-                add_qname(value.text)
+                add_qname(value.text, False)
         text = elem.text
         if isinstance(text, QName) and text.text not in qnames:
-            add_qname(text.text)
+            add_qname(text.text, False)
     return qnames, namespaces
 
 def _serialize_xml(write, elem, qnames, namespaces,