encoding during elementtree serialization

[Chris McDonough]

ElementTree's XML serialization routine implied by tree._write(file, 
node, encoding, namespaces looks like this (elided):

     def _write(self, file, node, encoding, namespaces):
         # write XML to file
         tag = node.tag
         if tag is Comment:
             file.write("<!-- %s -->" % _escape_cdata(node.text, encoding))
         elif tag is ProcessingInstruction:
             file.write("<?%s?>" % _escape_cdata(node.text, encoding))
         else:
...
             file.write("<" + _encode(tag, encoding))
             if items or xmlns_items:
                 items.sort() # lexical order

Note that "_escape_cdata" (which also performs encoding) and "_encode" 
are called for pcdata (and attribute values) only, but not for the tag 
literals like "<" and "<?%s?>".

In some profiling I've done, I believe encoding during recursion makes 
serialization slightly slower than it could be if we could get away with 
not encoding any pcdata or attribute values during recursion.

Instead, we might be able to get away with encoding everything just once 
at the end.  But I don't know if this is kosher.  Is there any reason to 
not also encode tag literals and quotation marks that are attribute 
containers, just once, at the end of serialization?

Even if that's not acceptable in general because tag literals cannot be 
encoded, would it be acceptable for "ascii-compatible" encodings like 
utf-8, latin-1, and friends?

Something like:

def _escape_cdata(text, encoding=None, replace=string.replace):
     # doesn't do any encoding
     text = replace(text, "&", "&amp;")
     text = replace(text, "<", "&lt;")
     text = replace(text, ">", "&gt;")
     return text

class _ElementInterface:

     ...

     def write(self, file, encoding="us-ascii"):
         assert self._root is not None
         if not hasattr(file, "write"):
             file = open(file, "wb")
         if not encoding:
             encoding = "us-ascii"
         elif encoding != "utf-8" and encoding != "us-ascii":
             file.write("<?xml version='1.0' encoding='%s'?>\n" % encoding)
         tmp = StringIO()
         self._write(tmp, self._root, encoding, {})
         file.write(tmp.getvalue().encode(encoding))


     def _write(self, file, node, encoding, namespaces):
         # write XML to file
         tag = node.tag
         if tag is Comment:
             file.write("<!-- %s -->" % _escape_cdata(node.text, encoding))
         elif tag is ProcessingInstruction:
             file.write("<?%s?>" % _escape_cdata(node.text, encoding))
         else:
             items = node.items()
             xmlns_items = [] # new namespaces in this scope
             try:
                 if isinstance(tag, QName) or tag[:1] == "{":
                     tag, xmlns = fixtag(tag, namespaces)
                     if xmlns: xmlns_items.append(xmlns)
             except TypeError:
                 _raise_serialization_error(tag)
             file.write("<" + tag)


I smell the mention of a Byte Order Mark coming on. ;-)
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