Reuse a prefix in scope for an unprefixed attribute in a namespace

A prefix is only invented if no prefix in scope is bound to the namespace
of the attribute.  The reverse mapping is built only when such attribute
is encountered, so there is no cost in the common case.  The counter of
invented prefixes is not reset for every attribute.

Author: serhiy-storchaka

Lib/test/test_minidom.py

@@ -597,6 +597,44 @@ def testWriteXMLAttributeNamespacePrefix(self):
                 '<root xmlns:ns0="http://xml.python.org/ns" '
                 'xmlns:ns1="http://xml.python.org/ns2" '
                 'ns0:attr="value" ns1:attr2="value2"/>')
+        # The same namespace gets the same prefix.
+        root.setAttributeNS("http://xml.python.org/ns", "attr3", "value3")
+        self.assertEqual(dom.documentElement.toxml(),
+                '<root xmlns:ns0="http://xml.python.org/ns" '
+                'xmlns:ns1="http://xml.python.org/ns2" '
+                'ns0:attr="value" ns1:attr2="value2" ns0:attr3="value3"/>')
+        dom.unlink()
+
+    def testWriteXMLAttributeNamespacePrefixReused(self):
+        # A prefix already bound to the namespace of the attribute is used.
+        dom = Document()
+        root = dom.appendChild(
+            dom.createElementNS("http://xml.python.org/ns", "p:root"))
+        root.setAttributeNS("http://xml.python.org/ns", "attr", "value")
+        self.assertEqual(dom.documentElement.toxml(),
+                '<p:root xmlns:p="http://xml.python.org/ns" p:attr="value"/>')
+        # The prefix can be bound for an ancestor.
+        child = root.appendChild(dom.createElement("child"))
+        child.setAttributeNS("http://xml.python.org/ns", "attr", "value")
+        self.assertEqual(child.toxml(), '<child p:attr="value"/>')
+        # The prefix bound for a preceding attribute is reused.
+        root.setAttributeNS("http://xml.python.org/ns3", "q:attr3", "value3")
+        root.setAttributeNS("http://xml.python.org/ns3", "attr4", "value4")
+        self.assertEqual(dom.documentElement.toxml(),
+                '<p:root xmlns:p="http://xml.python.org/ns" '
+                'xmlns:q="http://xml.python.org/ns3" '
+                'p:attr="value" q:attr3="value3" q:attr4="value4">'
+                '<child p:attr="value"/></p:root>')
+        root.removeAttributeNS("http://xml.python.org/ns3", "attr3")
+        root.removeAttributeNS("http://xml.python.org/ns3", "attr4")
+        # The prefix must not be taken by an explicit declaration.
+        root.setAttributeNS(xml.dom.XMLNS_NAMESPACE, "xmlns:ns0", "other")
+        root.setAttributeNS("http://xml.python.org/ns2", "attr2", "value2")
+        self.assertEqual(dom.documentElement.toxml(),
+                '<p:root xmlns:p="http://xml.python.org/ns" '
+                'xmlns:ns1="http://xml.python.org/ns2" '
+                'p:attr="value" xmlns:ns0="other" ns1:attr2="value2">'
+                '<child p:attr="value"/></p:root>')
         dom.unlink()
 
     def testWriteXMLXMLPrefix(self):

Lib/xml/dom/minidom.py

@@ -420,20 +420,29 @@ def _fixup_namespaces(element, nsmap):
         declarations.append(("xmlns", ""))
 
     items = []
+    prefixes = None  # namespace URI -> prefix, built only when needed
+    n = 0
     for name, value, attr_uri, attr in entries:
         if attr_uri is not None:
             # Unprefixed attributes are in no namespace, so an attribute
             # in a namespace always needs a prefix.
             prefix, _, _ = name.rpartition(':')
             if not prefix:
-                n = 0
-                while nsmap.get("ns%d" % n) is not None:
-                    n += 1
-                prefix = "ns%d" % n
+                # Reuse a prefix bound to the namespace, or invent one.
+                if prefixes is None:
+                    prefixes = {u: p for p, u in nsmap.items()
+                                if p is not None}
+                prefix = prefixes.get(attr_uri)
+                if prefix is None:
+                    while nsmap.get("ns%d" % n) is not None:
+                        n += 1
+                    prefix = "ns%d" % n
                 name = "%s:%s" % (prefix, attr.localName)
             if nsmap.get(prefix) != attr_uri:
                 nsmap = _bind_namespace(nsmap, inherited, prefix, attr_uri)
                 declarations.append(("xmlns:" + prefix, attr_uri))
+                if prefixes is not None:
+                    prefixes[attr_uri] = prefix
         items.append((name, value))
 
     if declarations: