BeautifulSoup Document without parser - Currently broken!

soup.py

from bs4.element import Tag, NavigableString


class Doc(Tag):
    """
    Dumby Document Root.
    
    This class provides a document root object without a parser.
    The document will need to be built mannually by adding Tags,
    NavigableStrings, Comments and the like. This class assumes
    HTML (not XML) and offers no options.
    
    By default it serializes to a string using the "html" formatter
    and "pretty_print". It only outputs Unicode strings.
    """
    
    def __init__(self):
        self.name = "document"
        self.hidden = True
        self.is_xml = False
        self.builder = None
        self.parser_class = None
        self.namespace = None
        self.prefix = None
        self.attrs = {}
        self.contents = []
    
    def new_tag(self, name, namespace=None, nsprefix=None, **attrs):
        """Create a new tag associated with this soup."""
        return Tag(None, self.builder, name, namespace, nsprefix, attrs)

    def new_string(self, s, subclass=NavigableString):
        """Create a new NavigableString associated with this soup."""
        return subclass(s)
    
    def insert_before(self, successor):
        raise NotImplementedError("Doc objects don't support insert_before().")

    def insert_after(self, successor):
        raise NotImplementedError("Doc objects don't support insert_after().")
    
    def decode(self, pretty_print=True, formatter="html"):
        """
        Return a Unicode string of this document.
        """
        indent_level = 0 if pretty_print else None
        return super(Doc, self).decode(indent_level, None, formatter)

test_soup.py

from unittest import TestCase
import soup


class TestSoup(TestCase):
    
    def test_doc(self):
        doc = soup.Doc()
        self.assertEqual(doc.decode(), '')
    
    def test_append(self):
        doc = soup.Doc()
        p = doc.new_tag('p', attrs={'class':'foo'})
        text = doc.new_string('Some paragraph text')
        p.append(text)
        doc.append(p)
        self.assertEqual(p.parent, doc)
        self.assertEqual(
            doc.prettify(),
            '<p class="foo">Some paragraph text</p>'
        )