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

@@ -165,6 +165,7 @@ def __init__(self, element_or_tree, content_handler):
         from xml.sax.xmlreader import AttributesNSImpl as attr_class
         self._attr_class = attr_class
         self._empty_attributes = attr_class({}, {})
+        self._prefix_map = dict((v, k) for (k, v) in element.nsmap.items())
 
     def saxify(self):
         self._content_handler.startDocument()
@@ -238,8 +239,17 @@ def _build_qname(self, ns_uri, local_name, prefixes, new_prefixes):
         try:
             prefix = prefixes[ns_uri]
         except KeyError:
-            prefix = prefixes[ns_uri] = 'ns%02d' % len(prefixes)
+            # New prefix
+            try:
+                prefix = self._prefix_map[ns_uri]
+            except KeyError:
+                # Unknown prefix, generate one
+                prefix = prefixes[ns_uri] = 'ns%02d' % len(prefixes)
             new_prefixes.append( (prefix, ns_uri) )
+
+        if prefix is None:
+            # Default namespace
+            return local_name
         return prefix + ':' + local_name
 
 def saxify(element_or_tree, content_handler):