ElementPath with default namespace support

ElementPath-xpath_tokenizer-original.py

def xpath_tokenizer(pattern, namespaces=None):
    for token in xpath_tokenizer_re.findall(pattern):
        tag = token[1]
        if tag and tag[0] != "{" and ":" in tag:
            try:    
                prefix, uri = tag.split(":", 1)
                if not namespaces:
                    raise KeyError
                yield token[0], "{%s}%s" % (namespaces[prefix], uri)
            except KeyError:
                raise SyntaxError("prefix %r not found in prefix map" % prefix)
        else:
            yield token

ElementPath-xpath_tokenizer-updated.py

def xpath_tokenizer(pattern, namespaces=None):
    for token in xpath_tokenizer_re.findall(pattern):
        tag = token[1]
        if tag and tag[0] != "{":
            try:    
                if ':' in tag:
                    prefix, uri = tag.split(":", 1)
                else:
                    prefix, uri = '', tag
                if not namespaces:
                    raise KeyError
                yield token[0], "{%s}%s" % (namespaces[prefix], uri)
            except KeyError:
                raise SyntaxError("prefix %r not found in prefix map" % prefix)
        else:
            yield token

example.py

from xml.etree import ElementTree as ET
doc = ET.fromstring('''
<root xmlns="http://example.com" xmlns:pre="http://example.com/prefix">
  <a>1</a>
  <pre:b>2</pre:b>
</root>
''')
nsmap = {'': 'http://example.com', 'pre':'http://example.com/prefix'}
doc.find('.//a')  # finds nothing
doc.find('.//a', namespaces=nsmap)  # finds nothing
doc.find('.//{http://example.com}a')  # works, but yuck
doc.find('.//pre:b', namespaces=nsmap)  # this works, though

ElementPath-xpath_tokenizer-original.py is the function from the xml.etree.ElementPath module in Python 3.5. ElementPath-xpath_tokenizer-updated.py is a version that also adds URIs for the default namespace.