Let ElementTreeProducer use the available namespaces

ElementTreeProducer would ignore the namespace prefixes that were available in the element tree, and always generate new prefixes like ns00, ns01 etc.

CHANGES.txt

@@ -12,6 +12,10 @@ Bugs fixed
   and the parser participates in a reference cycle.
   Original patch by Julien Greard.
 
+* ElementTreeProducer no longer ignores the namespace prefixes that were available
+  in the element tree, and now only generates nsXX prefixes if undefined prefixes
+  are encountered.
+
 
 4.2.1 (2018-03-21)
 ==================

src/lxml/sax.py

@@ -200,7 +200,19 @@ def _recursive_saxify(self, element, prefixes):
                 content_handler.characters(element.tail)
             return
 
+        # Get a new copy in this call, so changes doesn't propagate upwards
+        prefixes = prefixes.copy()
         new_prefixes = []
+        for prefix, ns_uri in element.nsmap.items():
+            if prefixes.get(prefix) != ns_uri:
+                # New or updated namespace
+                new_prefixes.append( (prefix, ns_uri) )
+                if ns_uri in prefixes and prefixes[ns_uri] is None:
+                    # This URI is the default URI,
+                    # don't use the assigned prefix
+                    continue
+                prefixes[ns_uri] = prefix
+
         build_qname = self._build_qname
         attribs = element.items()
         if attribs:
@@ -210,13 +222,13 @@ def _recursive_saxify(self, element, prefixes):
                 attr_ns_tuple = _getNsTag(attr_ns_name)
                 attr_values[attr_ns_tuple] = value
                 attr_qnames[attr_ns_tuple] = build_qname(
-                    attr_ns_tuple[0], attr_ns_tuple[1], prefixes, new_prefixes)
+                    attr_ns_tuple[0], attr_ns_tuple[1], prefixes)
             sax_attributes = self._attr_class(attr_values, attr_qnames)
         else:
             sax_attributes = self._empty_attributes
 
         ns_uri, local_name = _getNsTag(tag)
-        qname = build_qname(ns_uri, local_name, prefixes, new_prefixes)
+        qname = build_qname(ns_uri, local_name, prefixes)
 
         for prefix, uri in new_prefixes:
             content_handler.startPrefixMapping(prefix, uri)
@@ -232,14 +244,15 @@ def _recursive_saxify(self, element, prefixes):
         if element.tail:
             content_handler.characters(element.tail)
 
-    def _build_qname(self, ns_uri, local_name, prefixes, new_prefixes):
+    def _build_qname(self, ns_uri, local_name, prefixes):
         if ns_uri is None:
             return local_name
-        try:
-            prefix = prefixes[ns_uri]
-        except KeyError:
-            prefix = prefixes[ns_uri] = 'ns%02d' % len(prefixes)
-            new_prefixes.append( (prefix, ns_uri) )
+
+        prefix = prefixes[ns_uri]
+
+        if prefix is None:
+            # Default namespace
+            return local_name
         return prefix + ':' + local_name
 
 def saxify(element_or_tree, content_handler):

src/lxml/tests/test_sax.py

@@ -128,6 +128,37 @@ def test_element_sax_ns(self):
         self.assertEqual(0,
                          len(root))
 
+    def test_element_sax_ns_prefix(self):
+        # The name of the prefix should be preserved
+        tree = self.parse('<a:a xmlns:a="blaA"><b/><c:c xmlns:c="blaC">'
+                          '<d/></c:c></a:a>')
+        a = tree.getroot()
+
+        self.assertEqual(b'<a:a xmlns:a="blaA"><b/><c:c xmlns:c="blaC">'
+                         b'<d/></c:c></a:a>',
+                         self._saxify_serialize(a))
+
+    def test_element_sax_default_ns_prefix(self):
+        # Default prefixes should also not get a generated prefix
+        tree = self.parse('<a xmlns="blaA"><b/><c:c xmlns:c="blaC">'
+                          '<d/></c:c></a>')
+        a = tree.getroot()
+
+        self.assertEqual(b'<a xmlns="blaA"><b/><c:c xmlns:c="blaC">'
+                         b'<d/></c:c></a>',
+                         self._saxify_serialize(a))
+
+    def test_element_sax_unknown_ns_prefix(self):
+        # Make an element with an unregister prefix
+        tree = self.parse('<a xmlns="blaA"><b/><c:c xmlns:c="blaC">'
+                          '<d/></c:c></a>')
+        a = tree.getroot()
+        a.append(a.makeelement('{blaE}e'))
+
+        self.assertEqual(b'<a xmlns="blaA"><b/><c:c xmlns:c="blaC">'
+                         b'<d/></c:c><ns0:e xmlns:ns0="blaE"/></a>',
+                         self._saxify_serialize(a))
+
     def test_etree_sax_handler_default_ns(self):
         handler = sax.ElementTreeContentHandler()
         handler.startDocument()