Keep track of the namespace abbreviations found while parsing the document. This makes select() work most of the time without requiring a value for 'namespaces'.

2 files changed, 44 insertions, 6 deletions

diff --git a/bs4/builder/__init__.py b/bs4/builder/__init__.py
index c9e3f3d..610d42f 100644
--- a/bs4/builder/__init__.py
+++ b/bs4/builder/__init__.py
@@ -102,6 +102,12 @@ class TreeBuilder(object):
     def __init__(self):
         self.soup = None
 
+    def initialize_soup(self, soup):
+        """The BeautifulSoup object has been initialized and is now
+        being associated with the TreeBuilder.
+        """
+        self.soup = soup
+        
     def reset(self):
         pass
 
diff --git a/bs4/builder/_lxml.py b/bs4/builder/_lxml.py
index 4a0f7de..7debf56 100644
--- a/bs4/builder/_lxml.py
+++ b/bs4/builder/_lxml.py
@@ -32,6 +32,10 @@ from bs4.dammit import EncodingDetector
 
 LXML = 'lxml'
 
+def _invert(d):
+    "Invert a dictionary."
+    return dict((v,k) for k, v in d.items())
+
 class LXMLTreeBuilderForXML(TreeBuilder):
     DEFAULT_PARSER_CLASS = etree.XMLParser
 
@@ -48,8 +52,30 @@ class LXMLTreeBuilderForXML(TreeBuilder):
 
     # This namespace mapping is specified in the XML Namespace
     # standard.
-    DEFAULT_NSMAPS = {'http://www.w3.org/XML/1998/namespace' : "xml"}
+    DEFAULT_NSMAPS = dict(xml='http://www.w3.org/XML/1998/namespace')
+
+    DEFAULT_NSMAPS_INVERTED = _invert(DEFAULT_NSMAPS)
+
+    def initialize_soup(self, soup):
+        """Let the BeautifulSoup object know about the standard namespace
+        mapping.
+        """
+        super(LXMLTreeBuilderForXML, self).initialize_soup(soup)
+        self._register_namespaces(self.DEFAULT_NSMAPS)
+
+    def _register_namespaces(self, mapping):
+        """Let the BeautifulSoup object know about namespaces encountered
+        while parsing the document.
 
+        This might be useful later on when creating CSS selectors.
+        """
+        for key, value in mapping.items():
+            if key not in self.soup._namespaces:
+                    # Let the BeautifulSoup object know about a new namespace.
+                    # If there are multiple namespaces defined with the same
+                    # prefix, the first one in the document takes precedence.
+                    self.soup._namespaces[key] = value
+        
     def default_parser(self, encoding):
         # This can either return a parser object or a class, which
         # will be instantiated with default arguments.
@@ -75,8 +101,8 @@ class LXMLTreeBuilderForXML(TreeBuilder):
         if empty_element_tags is not None:
             self.empty_element_tags = set(empty_element_tags)
         self.soup = None
-        self.nsmaps = [self.DEFAULT_NSMAPS]
-
+        self.nsmaps = [self.DEFAULT_NSMAPS_INVERTED]
+        
     def _getNsTag(self, tag):
         # Split the namespace URL out of a fully-qualified lxml tag
         # name. Copied from lxml's src/lxml/sax.py.
@@ -144,7 +170,7 @@ class LXMLTreeBuilderForXML(TreeBuilder):
             raise ParserRejectedMarkup(str(e))
 
     def close(self):
-        self.nsmaps = [self.DEFAULT_NSMAPS]
+        self.nsmaps = [self.DEFAULT_NSMAPS_INVERTED]
 
     def start(self, name, attrs, nsmap={}):
         # Make sure attrs is a mutable dict--lxml may send an immutable dictproxy.
@@ -158,8 +184,14 @@ class LXMLTreeBuilderForXML(TreeBuilder):
                 self.nsmaps.append(None)
         elif len(nsmap) > 0:
             # A new namespace mapping has come into play.
-            inverted_nsmap = dict((value, key) for key, value in nsmap.items())
-            self.nsmaps.append(inverted_nsmap)
+
+            # First, Let the BeautifulSoup object know about it.
+            self._register_namespaces(nsmap)
+
+            # Then, add it to our running list of inverted namespace
+            # mappings.
+            self.nsmaps.append(_invert(nsmap))
+
             # Also treat the namespace mapping as a set of attributes on the
             # tag, so we can recreate it later.
             attrs = attrs.copy()