lxml.html — lxml documentation

# Source code for lxml.html

    # Copyright (c) 2004 Ian Bicking. All rights reserved.
    #
    # Redistribution and use in source and binary forms, with or without
    # modification, are permitted provided that the following conditions are
    # met:
    #
    # 1. Redistributions of source code must retain the above copyright
    # notice, this list of conditions and the following disclaimer.
    #
    # 2. Redistributions in binary form must reproduce the above copyright
    # notice, this list of conditions and the following disclaimer in
    # the documentation and/or other materials provided with the
    # distribution.
    #
    # 3. Neither the name of Ian Bicking nor the names of its contributors may
    # be used to endorse or promote products derived from this software
    # without specific prior written permission.
    #
    # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    # A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL IAN BICKING OR
    # CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
    # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
    # PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
    # PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
    # LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
    # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
    # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

    """The ``lxml.html`` tool set for HTML handling.
    """

    __all__ = [
        'document_fromstring', 'fragment_fromstring', 'fragments_fromstring', 'fromstring',
        'tostring', 'Element', 'defs', 'open_in_browser', 'submit_form',
        'find_rel_links', 'find_class', 'make_links_absolute',
        'resolve_base_href', 'iterlinks', 'rewrite_links', 'parse']

    import copy
    import re

    from collections.abc import MutableMapping, MutableSet
    from functools import partial
    from urllib.parse import urljoin

    from .. import etree
    from . import defs
    from ._setmixin import SetMixin

    def __fix_docstring(s):
        # TODO: remove and clean up doctests
        if not s:
            return s
        sub = re.compile(r"^(\s*)u'", re.M).sub
        return sub(r"\1'", s)

    XHTML_NAMESPACE = "http://www.w3.org/1999/xhtml"

    _rel_links_xpath = etree.XPath("descendant-or-self::a[@rel]|descendant-or-self::x:a[@rel]",
                                   namespaces={'x':XHTML_NAMESPACE})
    _options_xpath = etree.XPath("descendant-or-self::option|descendant-or-self::x:option",
                                 namespaces={'x':XHTML_NAMESPACE})
    _forms_xpath = etree.XPath("descendant-or-self::form|descendant-or-self::x:form",
                               namespaces={'x':XHTML_NAMESPACE})
    #_class_xpath = etree.XPath(r"descendant-or-self::*[regexp:match(@class, concat('\b', $class_name, '\b'))]", {'regexp': 'http://exslt.org/regular-expressions'})
    _class_xpath = etree.XPath("descendant-or-self::*[@class and contains(concat(' ', normalize-space(@class), ' '), concat(' ', $class_name, ' '))]")
    _id_xpath = etree.XPath("descendant-or-self::*[@id=$id]")
    _collect_string_content = etree.XPath("string()")
    _iter_css_urls = re.compile(r'url\(('+'["][^"]*["]|'+"['][^']*[']|"+r'[^)]*)\)', re.I).finditer
    _iter_css_imports = re.compile(r'@import "(.*?)"').finditer
    _label_xpath = etree.XPath("//label[@for=$id]|//x:label[@for=$id]",
                               namespaces={'x':XHTML_NAMESPACE})
    _archive_re = re.compile(r'[^ ]+')
    _parse_meta_refresh_url = re.compile(
        r'[^;=]*;\s*(?:url\s*=\s*)?(?P<url>.*)$', re.I).search

    def _unquote_match(s, pos):
        if s[:1] == '"' and s[-1:] == '"' or s[:1] == "'" and s[-1:] == "'":
            return s[1:-1], pos+1
        else:
            return s,pos

    def _transform_result(typ, result):
        """Convert the result back into the input type.
        """
        if issubclass(typ, bytes):
            return tostring(result, encoding='utf-8')
        elif issubclass(typ, str):
            return tostring(result, encoding='unicode')
        else:
            return result

    def _nons(tag):
        if isinstance(tag, str):
            if tag[0] == '{' and tag[1:len(XHTML_NAMESPACE)+1] == XHTML_NAMESPACE:
                return tag.split('}')[-1]
        return tag

    class Classes(MutableSet):
        """Provides access to an element's class attribute as a set-like collection.
        Usage::

            >>> el = fromstring('<p class="hidden large">Text</p>')
            >>> classes = el.classes  # or: classes = Classes(el.attrib)
            >>> classes |= ['block', 'paragraph']
            >>> el.get('class')
            'hidden large block paragraph'
            >>> classes.toggle('hidden')
            False
            >>> el.get('class')
            'large block paragraph'
            >>> classes -= ('some', 'classes', 'block')
            >>> el.get('class')
            'large paragraph'
        """
        def __init__(self, attributes):
            self._attributes = attributes
            self._get_class_value = partial(attributes.get, 'class', '')

        def add(self, value):
            """
            Add a class.

            This has no effect if the class is already present.
            """
            if not value or re.search(r'\s', value):
                raise ValueError("Invalid class name: %r" % value)
            classes = self._get_class_value().split()
            if value in classes:
                return
            classes.append(value)
            self._attributes['class'] = ' '.join(classes)

        def discard(self, value):
            """
            Remove a class if it is currently present.

            If the class is not present, do nothing.
            """
            if not value or re.search(r'\s', value):
                raise ValueError("Invalid class name: %r" % value)
            classes = [name for name in self._get_class_value().split()
                       if name != value]
            if classes:
                self._attributes['class'] = ' '.join(classes)
            elif 'class' in self._attributes:
                del self._attributes['class']

        def remove(self, value):
            """
            Remove a class; it must currently be present.

            If the class is not present, raise a KeyError.
            """
            if not value or re.search(r'\s', value):
                raise ValueError("Invalid class name: %r" % value)
            super().remove(value)

        def __contains__(self, name):
            classes = self._get_class_value()
            return name in classes and name in classes.split()

        def __iter__(self):
            return iter(self._get_class_value().split())

        def __len__(self):
            return len(self._get_class_value().split())

        # non-standard methods

        def update(self, values):
            """
            Add all names from 'values'.
            """
            classes = self._get_class_value().split()
            extended = False
            for value in values:
                if value not in classes:
                    classes.append(value)
                    extended = True
            if extended:
                self._attributes['class'] = ' '.join(classes)

        def toggle(self, value):
            """
            Add a class name if it isn't there yet, or remove it if it exists.

            Returns true if the class was added (and is now enabled) and
            false if it was removed (and is now disabled).
            """
            if not value or re.search(r'\s', value):
                raise ValueError("Invalid class name: %r" % value)
            classes = self._get_class_value().split()
            try:
                classes.remove(value)
                enabled = False
            except ValueError:
                classes.append(value)
                enabled = True
            if classes:
                self._attributes['class'] = ' '.join(classes)
            else:
                del self._attributes['class']
            return enabled

    class HtmlMixin:

        def set(self, key, value=None):
            """set(self, key, value=None)

            Sets an element attribute.  If no value is provided, or if the value is None,
            creates a 'boolean' attribute without value, e.g. "<form novalidate></form>"
            for ``form.set('novalidate')``.
            """
            super().set(key, value)

        @property
        def classes(self):
            """
            A set-like wrapper around the 'class' attribute.
            """
            return Classes(self.attrib)

        @classes.setter
        def classes(self, classes):
            assert isinstance(classes, Classes)  # only allow "el.classes |= ..." etc.
            value = classes._get_class_value()
            if value:
                self.set('class', value)
            elif self.get('class') is not None:
                del self.attrib['class']

        @property
        def base_url(self):
            """
            Returns the base URL, given when the page was parsed.

            Use with ``urlparse.urljoin(el.base_url, href)`` to get
            absolute URLs.
            """
            return self.getroottree().docinfo.URL

        @property
        def forms(self):
            """
            Return a list of all the forms
            """
            return _forms_xpath(self)

        @property
        def body(self):
            """
            Return the <body> element.  Can be called from a child element
            to get the document's head.
            """
            return self.xpath('//body|//x:body', namespaces={'x':XHTML_NAMESPACE})[0]

        @property
        def head(self):
            """
            Returns the <head> element.  Can be called from a child
            element to get the document's head.
            """
            return self.xpath('//head|//x:head', namespaces={'x':XHTML_NAMESPACE})[0]

        @property
        def label(self):
            """
            Get or set any <label> element associated with this element.
            """
            id = self.get('id')
            if not id:
                return None
            result = _label_xpath(self, id=id)
            if not result:
                return None
            else:
                return result[0]

        @label.setter
        def label(self, label):
            id = self.get('id')
            if not id:
                raise TypeError(
                    "You cannot set a label for an element (%r) that has no id"
                    % self)
            if _nons(label.tag) != 'label':
                raise TypeError(
                    "You can only assign label to a label element (not %r)"
                    % label)
            label.set('for', id)

        @label.deleter
        def label(self):
            label = self.label
            if label is not None:
                del label.attrib['for']

        def drop_tree(self):
            """
            Removes this element from the tree, including its children and
            text.  The tail text is joined to the previous element or
            parent.
            """
            parent = self.getparent()
            assert parent is not None
            if self.tail:
                previous = self.getprevious()
                if previous is None:
                    parent.text = (parent.text or '') + self.tail
                else:
                    previous.tail = (previous.tail or '') + self.tail
            parent.remove(self)

        def drop_tag(self):
            """
            Remove the tag, but not its children or text.  The children and text
            are merged into the parent.

            Example::

                >>> h = fragment_fromstring('<div>Hello <b>World!</b></div>')
                >>> h.find('.//b').drop_tag()
                >>> print(tostring(h, encoding='unicode'))
                <div>Hello World!</div>
            """
            parent = self.getparent()
            assert parent is not None
            previous = self.getprevious()
            if self.text and isinstance(self.tag, str):
                # not a Comment, etc.
                if previous is None:
                    parent.text = (parent.text or '') + self.text
                else:
                    previous.tail = (previous.tail or '') + self.text
            if self.tail:
                if len(self):
                    last = self[-1]
                    last.tail = (last.tail or '') + self.tail
                elif previous is None:
                    parent.text = (parent.text or '') + self.tail
                else:
                    previous.tail = (previous.tail or '') + self.tail
            index = parent.index(self)
            parent[index:index+1] = self[:]

        def find_rel_links(self, rel):
            """
            Find any links like ``<a rel="{rel}">...</a>``; returns a list of elements.
            """
            rel = rel.lower()
            return [el for el in _rel_links_xpath(self)
                    if el.get('rel').lower() == rel]

        def find_class(self, class_name):
            """
            Find any elements with the given class name.
            """
            return _class_xpath(self, class_name=class_name)

        def get_element_by_id(self, id, *default):
            """
            Get the first element in a document with the given id.  If none is
            found, return the default argument if provided or raise KeyError
            otherwise.

            Note that there can be more than one element with the same id,
            and this isn't uncommon in HTML documents found in the wild.
            Browsers return only the first match, and this function does
            the same.
            """
            try:
                # FIXME: should this check for multiple matches?
                # browsers just return the first one
                return _id_xpath(self, id=id)[0]
            except IndexError:
                if default:
                    return default[0]
                else:
                    raise KeyError(id)

        def text_content(self):
            """
            Return the text content of the tag (and the text in any children).
            """
            return _collect_string_content(self)

        def cssselect(self, expr, translator='html'):
            """
            Run the CSS expression on this element and its children,
            returning a list of the results.

            Equivalent to lxml.cssselect.CSSSelect(expr, translator='html')(self)
            -- note that pre-compiling the expression can provide a substantial
            speedup.
            """
            # Do the import here to make the dependency optional.
            from lxml.cssselect import CSSSelector
            return CSSSelector(expr, translator=translator)(self)

        ########################################
        ## Link functions
        ########################################

        def make_links_absolute(self, base_url=None, resolve_base_href=True,
                                handle_failures=None):
            """
            Make all links in the document absolute, given the
            ``base_url`` for the document (the full URL where the document
            came from), or if no ``base_url`` is given, then the ``.base_url``
            of the document.

            If ``resolve_base_href`` is true, then any ``<base href>``
            tags in the document are used *and* removed from the document.
            If it is false then any such tag is ignored.

            If ``handle_failures`` is None (default), a failure to process
            a URL will abort the processing.  If set to 'ignore', errors
            are ignored.  If set to 'discard', failing URLs will be removed.
            """
            if base_url is None:
                base_url = self.base_url
                if base_url is None:
                    raise TypeError(
                        "No base_url given, and the document has no base_url")
            if resolve_base_href:
                self.resolve_base_href()

            if handle_failures == 'ignore':
                def link_repl(href):
                    try:
                        return urljoin(base_url, href)
                    except ValueError:
                        return href
            elif handle_failures == 'discard':
                def link_repl(href):
                    try:
                        return urljoin(base_url, href)
                    except ValueError:
                        return None
            elif handle_failures is None:
                def link_repl(href):
                    return urljoin(base_url, href)
            else:
                raise ValueError(
                    "unexpected value for handle_failures: %r" % handle_failures)

            self.rewrite_links(link_repl)

        def resolve_base_href(self, handle_failures=None):
            """
            Find any ``<base href>`` tag in the document, and apply its
            values to all links found in the document.  Also remove the
            tag once it has been applied.

            If ``handle_failures`` is None (default), a failure to process
            a URL will abort the processing.  If set to 'ignore', errors
            are ignored.  If set to 'discard', failing URLs will be removed.
            """
            base_href = None
            basetags = self.xpath('//base[@href]|//x:base[@href]',
                                  namespaces={'x': XHTML_NAMESPACE})
            for b in basetags:
                base_href = b.get('href')
                b.drop_tree()
            if not base_href:
                return
            self.make_links_absolute(base_href, resolve_base_href=False,
                                     handle_failures=handle_failures)

        def iterlinks(self):
            """
            Yield (element, attribute, link, pos), where attribute may be None
            (indicating the link is in the text).  ``pos`` is the position
            where the link occurs; often 0, but sometimes something else in
            the case of links in stylesheets or style tags.

            Note: <base href> is *not* taken into account in any way.  The
            link you get is exactly the link in the document.

            Note: multiple links inside of a single text string or
            attribute value are returned in reversed order.  This makes it
            possible to replace or delete them from the text string value
            based on their reported text positions.  Otherwise, a
            modification at one text position can change the positions of
            links reported later on.
            """
            link_attrs = defs.link_attrs
            for el in self.iter(etree.Element):
                attribs = el.attrib
                tag = _nons(el.tag)
                if tag == 'object':
                    codebase = None
                    ## <object> tags have attributes that are relative to
                    ## codebase
                    if 'codebase' in attribs:
                        codebase = el.get('codebase')
                        yield (el, 'codebase', codebase, 0)
                    for attrib in ('classid', 'data'):
                        if attrib in attribs:
                            value = el.get(attrib)
                            if codebase is not None:
                                value = urljoin(codebase, value)
                            yield (el, attrib, value, 0)
                    if 'archive' in attribs:
                        for match in _archive_re.finditer(el.get('archive')):
                            value = match.group(0)
                            if codebase is not None:
                                value = urljoin(codebase, value)
                            yield (el, 'archive', value, match.start())
                else:
                    for attrib in link_attrs:
                        if attrib in attribs:
                            yield (el, attrib, attribs[attrib], 0)
                if tag == 'meta':
                    http_equiv = attribs.get('http-equiv', '').lower()
                    if http_equiv == 'refresh':
                        content = attribs.get('content', '')
                        match = _parse_meta_refresh_url(content)
                        url = (match.group('url') if match else content).strip()
                        # unexpected content means the redirect won't work, but we might
                        # as well be permissive and return the entire string.
                        if url:
                            url, pos = _unquote_match(
                                url, match.start('url') if match else content.find(url))
                            yield (el, 'content', url, pos)
                elif tag == 'param':
                    valuetype = el.get('valuetype') or ''
                    if valuetype.lower() == 'ref':
                        ## FIXME: while it's fine we *find* this link,
                        ## according to the spec we aren't supposed to
                        ## actually change the value, including resolving
                        ## it.  It can also still be a link, even if it
                        ## doesn't have a valuetype="ref" (which seems to be the norm)
                        ## http://www.w3.org/TR/html401/struct/objects.html#adef-valuetype
                        yield (el, 'value', el.get('value'), 0)
                elif tag == 'style' and el.text:
                    urls = [
                        # (start_pos, url)
                        _unquote_match(match.group(1), match.start(1))[::-1]
                        for match in _iter_css_urls(el.text)
                        ] + [
                        (match.start(1), match.group(1))
                        for match in _iter_css_imports(el.text)
                        ]
                    if urls:
                        # sort by start pos to bring both match sets back into order
                        # and reverse the list to report correct positions despite
                        # modifications
                        urls.sort(reverse=True)
                        for start, url in urls:
                            yield (el, None, url, start)
                if 'style' in attribs:
                    urls = list(_iter_css_urls(attribs['style']))
                    if urls:
                        # return in reversed order to simplify in-place modifications
                        for match in urls[::-1]:
                            url, start = _unquote_match(match.group(1), match.start(1))
                            yield (el, 'style', url, start)

        def rewrite_links(self, link_repl_func, resolve_base_href=True,
                          base_href=None):
            """
            Rewrite all the links in the document.  For each link
            ``link_repl_func(link)`` will be called, and the return value
            will replace the old link.

            Note that links may not be absolute (unless you first called
            ``make_links_absolute()``), and may be internal (e.g.,
            ``'#anchor'``).  They can also be values like
            ``'mailto:email'`` or ``'javascript:expr'``.

            If you give ``base_href`` then all links passed to
            ``link_repl_func()`` will take that into account.

            If the ``link_repl_func`` returns None, the attribute or
            tag text will be removed completely.
            """
            if base_href is not None:
                # FIXME: this can be done in one pass with a wrapper
                # around link_repl_func
                self.make_links_absolute(
                    base_href, resolve_base_href=resolve_base_href)
            elif resolve_base_href:
                self.resolve_base_href()

            for el, attrib, link, pos in self.iterlinks():
                new_link = link_repl_func(link.strip())
                if new_link == link:
                    continue
                if new_link is None:
                    # Remove the attribute or element content
                    if attrib is None:
                        el.text = ''
                    else:
                        del el.attrib[attrib]
                    continue

                if attrib is None:
                    new = el.text[:pos] + new_link + el.text[pos+len(link):]
                    el.text = new
                else:
                    cur = el.get(attrib)
                    if not pos and len(cur) == len(link):
                        new = new_link  # most common case
                    else:
                        new = cur[:pos] + new_link + cur[pos+len(link):]
                    el.set(attrib, new)

    class _MethodFunc:
        """
        An object that represents a method on an element as a function;
        the function takes either an element or an HTML string.  It
        returns whatever the function normally returns, or if the function
        works in-place (and so returns None) it returns a serialized form
        of the resulting document.
        """
        def __init__(self, name, copy=False, source_class=HtmlMixin):
            self.name = name
            self.copy = copy
            self.__doc__ = getattr(source_class, self.name).__doc__
        def __call__(self, doc, *args, **kw):
            result_type = type(doc)
            if isinstance(doc, (str, bytes)):
                if 'copy' in kw:
                    raise TypeError(
                        "The keyword 'copy' can only be used with element inputs to %s, not a string input" % self.name)
                doc = fromstring(doc, **kw)
            else:
                if 'copy' in kw:
                    make_a_copy = kw.pop('copy')
                else:
                    make_a_copy = self.copy
                if make_a_copy:
                    doc = copy.deepcopy(doc)
            meth = getattr(doc, self.name)
            result = meth(*args, **kw)
            # FIXME: this None test is a bit sloppy
            if result is None:
                # Then return what we got in
                return _transform_result(result_type, doc)
            else:
                return result

    find_rel_links = _MethodFunc('find_rel_links', copy=False)
    find_class = _MethodFunc('find_class', copy=False)
    make_links_absolute = _MethodFunc('make_links_absolute', copy=True)
    resolve_base_href = _MethodFunc('resolve_base_href', copy=True)
    iterlinks = _MethodFunc('iterlinks', copy=False)
    rewrite_links = _MethodFunc('rewrite_links', copy=True)

    class HtmlComment(HtmlMixin, etree.CommentBase):
        pass

    class HtmlElement(HtmlMixin, etree.ElementBase):
        pass

    class HtmlProcessingInstruction(HtmlMixin, etree.PIBase):
        pass

    class HtmlEntity(HtmlMixin, etree.EntityBase):
        pass

    class HtmlElementClassLookup(etree.CustomElementClassLookup):
        """A lookup scheme for HTML Element classes.

        To create a lookup instance with different Element classes, pass a tag
        name mapping of Element classes in the ``classes`` keyword argument and/or
        a tag name mapping of Mixin classes in the ``mixins`` keyword argument.
        The special key '*' denotes a Mixin class that should be mixed into all
        Element classes.
        """
        _default_element_classes = {}

        def __init__(self, classes=None, mixins=None):
            etree.CustomElementClassLookup.__init__(self)
            if classes is None:
                classes = self._default_element_classes.copy()
            if mixins:
                mixers = {}
                for name, value in mixins:
                    if name == '*':
                        for n in classes.keys():
                            mixers.setdefault(n, []).append(value)
                    else:
                        mixers.setdefault(name, []).append(value)
                for name, mix_bases in mixers.items():
                    cur = classes.get(name, HtmlElement)
                    bases = tuple(mix_bases + [cur])
                    classes[name] = type(cur.__name__, bases, {})
            self._element_classes = classes

        def lookup(self, node_type, document, namespace, name):
            if node_type == 'element':
                return self._element_classes.get(name.lower(), HtmlElement)
            elif node_type == 'comment':
                return HtmlComment
            elif node_type == 'PI':
                return HtmlProcessingInstruction
            elif node_type == 'entity':
                return HtmlEntity
            # Otherwise normal lookup
            return None

    ################################################################################
    # parsing
    ################################################################################

    _looks_like_full_html_unicode = re.compile(
        r'^\s*<(?:html|!doctype)', re.I).match
    _looks_like_full_html_bytes = re.compile(
        br'^\s*<(?:html|!doctype)', re.I).match

    def document_fromstring(html, parser=None, ensure_head_body=False, **kw):
        if parser is None:
            parser = html_parser
        value = etree.fromstring(html, parser, **kw)
        if value is None:
            raise etree.ParserError(
                "Document is empty")
        if ensure_head_body and value.find('head') is None:
            value.insert(0, Element('head'))
        if ensure_head_body and value.find('body') is None:
            value.append(Element('body'))
        return value

    def fragments_fromstring(html, no_leading_text=False, base_url=None,
                             parser=None, **kw):
        """Parses several HTML elements, returning a list of elements.

        The first item in the list may be a string.
        If no_leading_text is true, then it will be an error if there is
        leading text, and it will always be a list of only elements.

        base_url will set the document's base_url attribute
        (and the tree's docinfo.URL).
        """
        if parser is None:
            parser = html_parser
        # FIXME: check what happens when you give html with a body, head, etc.
        if isinstance(html, bytes):
            if not _looks_like_full_html_bytes(html):
                # can't use %-formatting in early Py3 versions
                html = (b'<html><body>' + html +
                        b'</body></html>')
        else:
            if not _looks_like_full_html_unicode(html):
                html = '<html><body>%s</body></html>' % html
        doc = document_fromstring(html, parser=parser, base_url=base_url, **kw)
        assert _nons(doc.tag) == 'html'
        bodies = [e for e in doc if _nons(e.tag) == 'body']
        assert len(bodies) == 1, ("too many bodies: %r in %r" % (bodies, html))
        body = bodies[0]
        elements = []
        if no_leading_text and body.text and body.text.strip():
            raise etree.ParserError(
                "There is leading text: %r" % body.text)
        if body.text and body.text.strip():
            elements.append(body.text)
        elements.extend(body)
        # FIXME: removing the reference to the parent artificial document
        # would be nice
        return elements

    def fragment_fromstring(html, create_parent=False, base_url=None,
                            parser=None, **kw):
        """
        Parses a single HTML element; it is an error if there is more than
        one element, or if anything but whitespace precedes or follows the
        element.

        If ``create_parent`` is true (or is a tag name) then a parent node
        will be created to encapsulate the HTML in a single element.  In this
        case, leading or trailing text is also allowed, as are multiple elements
        as result of the parsing.

        Passing a ``base_url`` will set the document's ``base_url`` attribute
        (and the tree's docinfo.URL).
        """
        if parser is None:
            parser = html_parser

        accept_leading_text = bool(create_parent)

        elements = fragments_fromstring(
            html, parser=parser, no_leading_text=not accept_leading_text,
            base_url=base_url, **kw)

        if create_parent:
            if not isinstance(create_parent, str):
                create_parent = 'div'
            new_root = Element(create_parent)
            if elements:
                if isinstance(elements[0], str):
                    new_root.text = elements[0]
                    del elements[0]
                new_root.extend(elements)
            return new_root

        if not elements:
            raise etree.ParserError('No elements found')
        if len(elements) > 1:
            raise etree.ParserError(
                "Multiple elements found (%s)"
                % ', '.join([_element_name(e) for e in elements]))
        el = elements[0]
        if el.tail and el.tail.strip():
            raise etree.ParserError(
                "Element followed by text: %r" % el.tail)
        el.tail = None
        return el

    def fromstring(html, base_url=None, parser=None, **kw):
        """
        Parse the html, returning a single element/document.

        This tries to minimally parse the chunk of text, without knowing if it
        is a fragment or a document.

        base_url will set the document's base_url attribute (and the tree's docinfo.URL)
        """
        if parser is None:
            parser = html_parser
        if isinstance(html, bytes):
            is_full_html = _looks_like_full_html_bytes(html)
        else:
            is_full_html = _looks_like_full_html_unicode(html)
        doc = document_fromstring(html, parser=parser, base_url=base_url, **kw)
        if is_full_html:
            return doc
        # otherwise, lets parse it out...
        bodies = doc.findall('body')
        if not bodies:
            bodies = doc.findall('{%s}body' % XHTML_NAMESPACE)
        if bodies:
            body = bodies[0]
            if len(bodies) > 1:
                # Somehow there are multiple bodies, which is bad, but just
                # smash them into one body
                for other_body in bodies[1:]:
                    if other_body.text:
                        if len(body):
                            body[-1].tail = (body[-1].tail or '') + other_body.text
                        else:
                            body.text = (body.text or '') + other_body.text
                    body.extend(other_body)
                    # We'll ignore tail
                    # I guess we are ignoring attributes too
                    other_body.drop_tree()
        else:
            body = None
        heads = doc.findall('head')
        if not heads:
            heads = doc.findall('{%s}head' % XHTML_NAMESPACE)
        if heads:
            # Well, we have some sort of structure, so lets keep it all
            head = heads[0]
            if len(heads) > 1:
                for other_head in heads[1:]:
                    head.extend(other_head)
                    # We don't care about text or tail in a head
                    other_head.drop_tree()
            return doc
        if body is None:
            return doc
        if (len(body) == 1 and (not body.text or not body.text.strip())
            and (not body[-1].tail or not body[-1].tail.strip())):
            # The body has just one element, so it was probably a single
            # element passed in
            return body[0]
        # Now we have a body which represents a bunch of tags which have the
        # content that was passed in.  We will create a fake container, which
        # is the body tag, except <body> implies too much structure.
        if _contains_block_level_tag(body):
            body.tag = 'div'
        else:
            body.tag = 'span'
        return body

    def parse(filename_or_url, parser=None, base_url=None, **kw):
        """
        Parse a filename, URL, or file-like object into an HTML document
        tree.  Note: this returns a tree, not an element.  Use
        ``parse(...).getroot()`` to get the document root.

        You can override the base URL with the ``base_url`` keyword.  This
        is most useful when parsing from a file-like object.
        """
        if parser is None:
            parser = html_parser
        return etree.parse(filename_or_url, parser, base_url=base_url, **kw)

    def _contains_block_level_tag(el):
        # FIXME: I could do this with XPath, but would that just be
        # unnecessarily slow?
        for el in el.iter(etree.Element):
            if _nons(el.tag) in defs.block_tags:
                return True
        return False

    def _element_name(el):
        if isinstance(el, etree.CommentBase):
            return 'comment'
        elif isinstance(el, str):
            return 'string'
        else:
            return _nons(el.tag)

    ################################################################################
    # form handling
    ################################################################################

    class FormElement(HtmlElement):
        """
        Represents a <form> element.
        """

        @property
        def inputs(self):
            """
            Returns an accessor for all the input elements in the form.

            See `InputGetter` for more information about the object.
            """
            return InputGetter(self)

        @property
        def fields(self):
            """
            Dictionary-like object that represents all the fields in this
            form.  You can set values in this dictionary to effect the
            form.
            """
            return FieldsDict(self.inputs)

        @fields.setter
        def fields(self, value):
            fields = self.fields
            prev_keys = fields.keys()
            for key, value in value.items():
                if key in prev_keys:
                    prev_keys.remove(key)
                fields[key] = value
            for key in prev_keys:
                if key is None:
                    # Case of an unnamed input; these aren't really
                    # expressed in form_values() anyway.
                    continue
                fields[key] = None

        def _name(self):
            if self.get('name'):
                return self.get('name')
            elif self.get('id'):
                return '#' + self.get('id')
            iter_tags = self.body.iter
            forms = list(iter_tags('form'))
            if not forms:
                forms = list(iter_tags('{%s}form' % XHTML_NAMESPACE))
            return str(forms.index(self))

        def form_values(self):
            """
            Return a list of tuples of the field values for the form.
            This is suitable to be passed to ``urllib.urlencode()``.
            """
            results = []
            for el in self.inputs:
                name = el.name
                if not name or 'disabled' in el.attrib:
                    continue
                tag = _nons(el.tag)
                if tag == 'textarea':
                    results.append((name, el.value))
                elif tag == 'select':
                    value = el.value
                    if el.multiple:
                        for v in value:
                            results.append((name, v))
                    elif value is not None:
                        results.append((name, el.value))
                else:
                    assert tag == 'input', (
                        "Unexpected tag: %r" % el)
                    if el.checkable and not el.checked:
                        continue
                    if el.type in ('submit', 'image', 'reset', 'file'):
                        continue
                    value = el.value
                    if value is not None:
                        results.append((name, el.value))
            return results

        @property
        def action(self):
            """
            Get/set the form's ``action`` attribute.
            """
            base_url = self.base_url
            action = self.get('action')
            if base_url and action is not None:
                return urljoin(base_url, action)
            else:
                return action

        @action.setter
        def action(self, value):
            self.set('action', value)

        @action.deleter
        def action(self):
            attrib = self.attrib
            if 'action' in attrib:
                del attrib['action']

        @property
        def method(self):
            """
            Get/set the form's method.  Always returns a capitalized
            string, and defaults to ``'GET'``
            """
            return self.get('method', 'GET').upper()

        @method.setter
        def method(self, value):
            self.set('method', value.upper())

    HtmlElementClassLookup._default_element_classes['form'] = FormElement

    def submit_form(form, extra_values=None, open_http=None):
        """
        Helper function to submit a form.  Returns a file-like object, as from
        ``urllib.urlopen()``.  This object also has a ``.geturl()`` function,
        which shows the URL if there were any redirects.

        You can use this like::

            form = doc.forms[0]
            form.inputs['foo'].value = 'bar' # etc
            response = form.submit()
            doc = parse(response)
            doc.make_links_absolute(response.geturl())

        To change the HTTP requester, pass a function as ``open_http`` keyword
        argument that opens the URL for you.  The function must have the following
        signature::

            open_http(method, URL, values)

        The action is one of 'GET' or 'POST', the URL is the target URL as a
        string, and the values are a sequence of ``(name, value)`` tuples with the
        form data.
        """
        values = form.form_values()
        if extra_values:
            if hasattr(extra_values, 'items'):
                extra_values = extra_values.items()
            values.extend(extra_values)
        if open_http is None:
            open_http = open_http_urllib
        if form.action:
            url = form.action
        else:
            url = form.base_url
        return open_http(form.method, url, values)

    def open_http_urllib(method, url, values):
        if not url:
            raise ValueError("cannot submit, no URL provided")
        ## FIXME: should test that it's not a relative URL or something
        try:
            from urllib import urlencode, urlopen
        except ImportError: # Python 3
            from urllib.request import urlopen
            from urllib.parse import urlencode
        if method == 'GET':
            if '?' in url:
                url += '&'
            else:
                url += '?'
            url += urlencode(values)
            data = None
        else:
            data = urlencode(values)
            if not isinstance(data, bytes):
                data = data.encode('ASCII')
        return urlopen(url, data)

    class FieldsDict(MutableMapping):

        def __init__(self, inputs):
            self.inputs = inputs
        def __getitem__(self, item):
            return self.inputs[item].value
        def __setitem__(self, item, value):
            self.inputs[item].value = value
        def __delitem__(self, item):
            raise KeyError(
                "You cannot remove keys from ElementDict")

        def keys(self):
            return self.inputs.keys()

        def __contains__(self, item):
            return item in self.inputs
        def __iter__(self):
            return iter(self.inputs.keys())
        def __len__(self):
            return len(self.inputs)

        def __repr__(self):
            return '<%s for form %s>' % (
                self.__class__.__name__,
                self.inputs.form._name())

    class InputGetter:

        """
        An accessor that represents all the input fields in a form.

        You can get fields by name from this, with
        ``form.inputs['field_name']``.  If there are a set of checkboxes
        with the same name, they are returned as a list (a `CheckboxGroup`
        which also allows value setting).  Radio inputs are handled
        similarly.  Use ``.keys()`` and ``.items()`` to process all fields
        in this way.

        You can also iterate over this to get all input elements.  This
        won't return the same thing as if you get all the names, as
        checkboxes and radio elements are returned individually.
        """

        def __init__(self, form):
            self.form = form

        def __repr__(self):
            return '<%s for form %s>' % (
                self.__class__.__name__,
                self.form._name())

        ## FIXME: there should be more methods, and it's unclear if this is
        ## a dictionary-like object or list-like object

        def __getitem__(self, name):
            fields = [field for field in self if field.name == name]
            if not fields:
                raise KeyError("No input element with the name %r" % name)

            input_type = fields[0].get('type')
            if input_type == 'radio' and len(fields) > 1:
                group = RadioGroup(fields)
                group.name = name
                return group
            elif input_type == 'checkbox' and len(fields) > 1:
                group = CheckboxGroup(fields)
                group.name = name
                return group
            else:
                # I don't like throwing away elements like this
                return fields[0]

        def __contains__(self, name):
            for field in self:
                if field.name == name:
                    return True
            return False

        def keys(self):
            """
            Returns all unique field names, in document order.

            :return: A list of all unique field names.
            """
            names = []
            seen = {None}
            for el in self:
                name = el.name
                if name not in seen:
                    names.append(name)
                    seen.add(name)
            return names

        def items(self):
            """
            Returns all fields with their names, similar to dict.items().

            :return: A list of (name, field) tuples.
            """
            items = []
            seen = set()
            for el in self:
                name = el.name
                if name not in seen:
                    seen.add(name)
                    items.append((name, self[name]))
            return items

        def __iter__(self):
            return self.form.iter('select', 'input', 'textarea')

        def __len__(self):
            return sum(1 for _ in self)

    class InputMixin:
        """
        Mix-in for all input elements (input, select, and textarea)
        """
        @property
        def name(self):
            """
            Get/set the name of the element
            """
            return self.get('name')

        @name.setter
        def name(self, value):
            self.set('name', value)

        @name.deleter
        def name(self):
            attrib = self.attrib
            if 'name' in attrib:
                del attrib['name']

        def __repr__(self):
            type_name = getattr(self, 'type', None)
            if type_name:
                type_name = ' type=%r' % type_name
            else:
                type_name = ''
            return '<%s %x name=%r%s>' % (
                self.__class__.__name__, id(self), self.name, type_name)

    class TextareaElement(InputMixin, HtmlElement):
        """
        ``<textarea>`` element.  You can get the name with ``.name`` and
        get/set the value with ``.value``
        """
        @property
        def value(self):
            """
            Get/set the value (which is the contents of this element)
            """
            content = self.text or ''
            if self.tag.startswith("{%s}" % XHTML_NAMESPACE):
                serialisation_method = 'xml'
            else:
                serialisation_method = 'html'
            for el in self:
                # it's rare that we actually get here, so let's not use ''.join()
                content += etree.tostring(
                    el, method=serialisation_method, encoding='unicode')
            return content

        @value.setter
        def value(self, value):
            del self[:]
            self.text = value

        @value.deleter
        def value(self):
            self.text = ''
            del self[:]

    HtmlElementClassLookup._default_element_classes['textarea'] = TextareaElement

    class SelectElement(InputMixin, HtmlElement):
        """
        ``<select>`` element.  You can get the name with ``.name``.

        ``.value`` will be the value of the selected option, unless this
        is a multi-select element (``<select multiple>``), in which case
        it will be a set-like object.  In either case ``.value_options``
        gives the possible values.

        The boolean attribute ``.multiple`` shows if this is a
        multi-select.
        """
        @property
        def value(self):
            """
            Get/set the value of this select (the selected option).

            If this is a multi-select, this is a set-like object that
            represents all the selected options.
            """
            if self.multiple:
                return MultipleSelectOptions(self)
            options = _options_xpath(self)

            try:
                selected_option = next(el for el in reversed(options) if el.get('selected') is not None)
            except StopIteration:
                try:
                    selected_option = next(el for el in options if el.get('disabled') is None)
                except StopIteration:
                    return None
            value = selected_option.get('value')
            if value is None:
                value = (selected_option.text or '').strip()
            return value

        @value.setter
        def value(self, value):
            if self.multiple:
                if isinstance(value, str):
                    raise TypeError("You must pass in a sequence")
                values = self.value
                values.clear()
                values.update(value)
                return
            checked_option = None
            if value is not None:
                for el in _options_xpath(self):
                    opt_value = el.get('value')
                    if opt_value is None:
                        opt_value = (el.text or '').strip()
                    if opt_value == value:
                        checked_option = el
                        break
                else:
                    raise ValueError(
                        "There is no option with the value of %r" % value)
            for el in _options_xpath(self):
                if 'selected' in el.attrib:
                    del el.attrib['selected']
            if checked_option is not None:
                checked_option.set('selected', '')

        @value.deleter
        def value(self):
            # FIXME: should del be allowed at all?
            if self.multiple:
                self.value.clear()
            else:
                self.value = None

        @property
        def value_options(self):
            """
            All the possible values this select can have (the ``value``
            attribute of all the ``<option>`` elements.
            """
            options = []
            for el in _options_xpath(self):
                value = el.get('value')
                if value is None:
                    value = (el.text or '').strip()
                options.append(value)
            return options

        @property
        def multiple(self):
            """
            Boolean attribute: is there a ``multiple`` attribute on this element.
            """
            return 'multiple' in self.attrib

        @multiple.setter
        def multiple(self, value):
            if value:
                self.set('multiple', '')
            elif 'multiple' in self.attrib:
                del self.attrib['multiple']

    HtmlElementClassLookup._default_element_classes['select'] = SelectElement

    class MultipleSelectOptions(SetMixin):
        """
        Represents all the selected options in a ``<select multiple>`` element.

        You can add to this set-like option to select an option, or remove
        to unselect the option.
        """

        def __init__(self, select):
            self.select = select

        @property
        def options(self):
            """
            Iterator of all the ``<option>`` elements.
            """
            return iter(_options_xpath(self.select))

        def __iter__(self):
            for option in self.options:
                if 'selected' in option.attrib:
                    opt_value = option.get('value')
                    if opt_value is None:
                        opt_value = (option.text or '').strip()
                    yield opt_value

        def add(self, item):
            for option in self.options:
                opt_value = option.get('value')
                if opt_value is None:
                    opt_value = (option.text or '').strip()
                if opt_value == item:
                    option.set('selected', '')
                    break
            else:
                raise ValueError(
                    "There is no option with the value %r" % item)

        def remove(self, item):
            for option in self.options:
                opt_value = option.get('value')
                if opt_value is None:
                    opt_value = (option.text or '').strip()
                if opt_value == item:
                    if 'selected' in option.attrib:
                        del option.attrib['selected']
                    else:
                        raise ValueError(
                            "The option %r is not currently selected" % item)
                    break
            else:
                raise ValueError(
                    "There is not option with the value %r" % item)

        def __repr__(self):
            return '<%s {%s} for select name=%r>' % (
                self.__class__.__name__,
                ', '.join([repr(v) for v in self]),
                self.select.name)

    class RadioGroup(list):
        """
        This object represents several ``<input type=radio>`` elements
        that have the same name.

        You can use this like a list, but also use the property
        ``.value`` to check/uncheck inputs.  Also you can use
        ``.value_options`` to get the possible values.
        """
        @property
        def value(self):
            """
            Get/set the value, which checks the radio with that value (and
            unchecks any other value).
            """
            for el in self:
                if 'checked' in el.attrib:
                    return el.get('value')
            return None

        @value.setter
        def value(self, value):
            checked_option = None
            if value is not None:
                for el in self:
                    if el.get('value') == value:
                        checked_option = el
                        break
                else:
                    raise ValueError("There is no radio input with the value %r" % value)
            for el in self:
                if 'checked' in el.attrib:
                    del el.attrib['checked']
            if checked_option is not None:
                checked_option.set('checked', '')

        @value.deleter
        def value(self):
            self.value = None

        @property
        def value_options(self):
            """
            Returns a list of all the possible values.
            """
            return [el.get('value') for el in self]

        def __repr__(self):
            return '%s(%s)' % (
                self.__class__.__name__,
                list.__repr__(self))

    class CheckboxGroup(list):
        """
        Represents a group of checkboxes (``<input type=checkbox>``) that
        have the same name.

        In addition to using this like a list, the ``.value`` attribute
        returns a set-like object that you can add to or remove from to
        check and uncheck checkboxes.  You can also use ``.value_options``
        to get the possible values.
        """
        @property
        def value(self):
            """
            Return a set-like object that can be modified to check or
            uncheck individual checkboxes according to their value.
            """
            return CheckboxValues(self)

        @value.setter
        def value(self, value):
            values = self.value
            values.clear()
            if not hasattr(value, '__iter__'):
                raise ValueError(
                    "A CheckboxGroup (name=%r) must be set to a sequence (not %r)"
                    % (self[0].name, value))
            values.update(value)

        @value.deleter
        def value(self):
            self.value.clear()

        @property
        def value_options(self):
            """
            Returns a list of all the possible values.
            """
            return [el.get('value') for el in self]

        def __repr__(self):
            return '%s(%s)' % (
                self.__class__.__name__, list.__repr__(self))

    class CheckboxValues(SetMixin):
        """
        Represents the values of the checked checkboxes in a group of
        checkboxes with the same name.
        """

        def __init__(self, group):
            self.group = group

        def __iter__(self):
            return iter([
                el.get('value')
                for el in self.group
                if 'checked' in el.attrib])

        def add(self, value):
            for el in self.group:
                if el.get('value') == value:
                    el.set('checked', '')
                    break
            else:
                raise KeyError("No checkbox with value %r" % value)

        def remove(self, value):
            for el in self.group:
                if el.get('value') == value:
                    if 'checked' in el.attrib:
                        del el.attrib['checked']
                    else:
                        raise KeyError(
                            "The checkbox with value %r was already unchecked" % value)
                    break
            else:
                raise KeyError(
                    "No checkbox with value %r" % value)

        def __repr__(self):
            return '<%s {%s} for checkboxes name=%r>' % (
                self.__class__.__name__,
                ', '.join([repr(v) for v in self]),
                self.group.name)

    class InputElement(InputMixin, HtmlElement):
        """
        Represents an ``<input>`` element.

        You can get the type with ``.type`` (which is lower-cased and
        defaults to ``'text'``).

        Also you can get and set the value with ``.value``

        Checkboxes and radios have the attribute ``input.checkable ==
        True`` (for all others it is false) and a boolean attribute
        ``.checked``.

        """

        ## FIXME: I'm a little uncomfortable with the use of .checked
        @property
        def value(self):
            """
            Get/set the value of this element, using the ``value`` attribute.

            Also, if this is a checkbox and it has no value, this defaults
            to ``'on'``.  If it is a checkbox or radio that is not
            checked, this returns None.
            """
            if self.checkable:
                if self.checked:
                    return self.get('value') or 'on'
                else:
                    return None
            return self.get('value')

        @value.setter
        def value(self, value):
            if self.checkable:
                if not value:
                    self.checked = False
                else:
                    self.checked = True
                    if isinstance(value, str):
                        self.set('value', value)
            else:
                self.set('value', value)

        @value.deleter
        def value(self):
            if self.checkable:
                self.checked = False
            else:
                if 'value' in self.attrib:
                    del self.attrib['value']

        @property
        def type(self):
            """
            Return the type of this element (using the type attribute).
            """
            return self.get('type', 'text').lower()

        @type.setter
        def type(self, value):
            self.set('type', value)

        @property
        def checkable(self):
            """
            Boolean: can this element be checked?
            """
            return self.type in ('checkbox', 'radio')

        @property
        def checked(self):
            """
            Boolean attribute to get/set the presence of the ``checked``
            attribute.

            You can only use this on checkable input types.
            """
            if not self.checkable:
                raise AttributeError('Not a checkable input type')
            return 'checked' in self.attrib

        @checked.setter
        def checked(self, value):
            if not self.checkable:
                raise AttributeError('Not a checkable input type')
            if value:
                self.set('checked', '')
            else:
                attrib = self.attrib
                if 'checked' in attrib:
                    del attrib['checked']

    HtmlElementClassLookup._default_element_classes['input'] = InputElement

    class LabelElement(HtmlElement):
        """
        Represents a ``<label>`` element.

        Label elements are linked to other elements with their ``for``
        attribute.  You can access this element with ``label.for_element``.
        """
        @property
        def for_element(self):
            """
            Get/set the element this label points to.  Return None if it
            can't be found.
            """
            id = self.get('for')
            if not id:
                return None
            return self.body.get_element_by_id(id)

        @for_element.setter
        def for_element(self, other):
            id = other.get('id')
            if not id:
                raise TypeError(
                    "Element %r has no id attribute" % other)
            self.set('for', id)

        @for_element.deleter
        def for_element(self):
            attrib = self.attrib
            if 'id' in attrib:
                del attrib['id']

    HtmlElementClassLookup._default_element_classes['label'] = LabelElement

    ############################################################
    ## Serialization
    ############################################################

    def html_to_xhtml(html):
        """Convert all tags in an HTML tree to XHTML by moving them to the
        XHTML namespace.
        """
        try:
            html = html.getroot()
        except AttributeError:
            pass
        prefix = "{%s}" % XHTML_NAMESPACE
        for el in html.iter(etree.Element):
            tag = el.tag
            if tag[0] != '{':
                el.tag = prefix + tag

    def xhtml_to_html(xhtml):
        """Convert all tags in an XHTML tree to HTML by removing their
        XHTML namespace.
        """
        try:
            xhtml = xhtml.getroot()
        except AttributeError:
            pass
        prefix = "{%s}" % XHTML_NAMESPACE
        prefix_len = len(prefix)
        for el in xhtml.iter(prefix + "*"):
            el.tag = el.tag[prefix_len:]

    # This isn't a general match, but it's a match for what libxml2
    # specifically serialises:
    __str_replace_meta_content_type = re.compile(
        r'<meta http-equiv="Content-Type"[^>]*>').sub
    __bytes_replace_meta_content_type = re.compile(
        br'<meta http-equiv="Content-Type"[^>]*>').sub

    def tostring(doc, pretty_print=False, include_meta_content_type=False,
                 encoding=None, method="html", with_tail=True, doctype=None):
        """Return an HTML string representation of the document.

        Note: if include_meta_content_type is true this will create a
        ``<meta http-equiv="Content-Type" ...>`` tag in the head;
        regardless of the value of include_meta_content_type any existing
        ``<meta http-equiv="Content-Type" ...>`` tag will be removed

        The ``encoding`` argument controls the output encoding (defaults to
        ASCII, with &#...; character references for any characters outside
        of ASCII).  Note that you can pass the name ``'unicode'`` as
        ``encoding`` argument to serialise to a Unicode string.

        The ``method`` argument defines the output method.  It defaults to
        'html', but can also be 'xml' for xhtml output, or 'text' to
        serialise to plain text without markup.

        To leave out the tail text of the top-level element that is being
        serialised, pass ``with_tail=False``.

        The ``doctype`` option allows passing in a plain string that will
        be serialised before the XML tree.  Note that passing in non
        well-formed content here will make the XML output non well-formed.
        Also, an existing doctype in the document tree will not be removed
        when serialising an ElementTree instance.

        Example::

            >>> from lxml import html
            >>> root = html.fragment_fromstring('<p>Hello<br>world!</p>')

            >>> html.tostring(root)
            b'<p>Hello<br>world!</p>'
            >>> html.tostring(root, method='html')
            b'<p>Hello<br>world!</p>'

            >>> html.tostring(root, method='xml')
            b'<p>Hello<br/>world!</p>'

            >>> html.tostring(root, method='text')
            b'Helloworld!'

            >>> html.tostring(root, method='text', encoding='unicode')
            u'Helloworld!'

            >>> root = html.fragment_fromstring('<div><p>Hello<br>world!</p>TAIL</div>')
            >>> html.tostring(root[0], method='text', encoding='unicode')
            u'Helloworld!TAIL'

            >>> html.tostring(root[0], method='text', encoding='unicode', with_tail=False)
            u'Helloworld!'

            >>> doc = html.document_fromstring('<p>Hello<br>world!</p>')
            >>> html.tostring(doc, method='html', encoding='unicode')
            u'<html><body><p>Hello<br>world!</p></body></html>'

            >>> print(html.tostring(doc, method='html', encoding='unicode',
            ...          doctype='<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"'
            ...                  ' "http://www.w3.org/TR/html4/strict.dtd">'))
            <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
            <html><body><p>Hello<br>world!</p></body></html>
        """
        html = etree.tostring(doc, method=method, pretty_print=pretty_print,
                              encoding=encoding, with_tail=with_tail,
                              doctype=doctype)
        if method == 'html' and not include_meta_content_type:
            if isinstance(html, str):
                html = __str_replace_meta_content_type('', html)
            else:
                html = __bytes_replace_meta_content_type(b'', html)
        return html

    tostring.__doc__ = __fix_docstring(tostring.__doc__)

    def open_in_browser(doc, encoding=None):
        """
        Open the HTML document in a web browser, saving it to a temporary
        file to open it.  Note that this does not delete the file after
        use.  This is mainly meant for debugging.
        """
        import os
        import webbrowser
        import tempfile
        if not isinstance(doc, etree._ElementTree):
            doc = etree.ElementTree(doc)
        handle, fn = tempfile.mkstemp(suffix='.html')
        f = os.fdopen(handle, 'wb')
        try:
            doc.write(f, method="html", encoding=encoding or doc.docinfo.encoding or "UTF-8")
        finally:
            # we leak the file itself here, but we should at least close it
            f.close()
        url = 'file://' + fn.replace(os.path.sep, '/')
        print(url)
        webbrowser.open(url)

    ################################################################################
    # configure Element class lookup
    ################################################################################

    class HTMLParser(etree.HTMLParser):
        """An HTML parser that is configured to return lxml.html Element
        objects.
        """
        def __init__(self, **kwargs):
            super().__init__(**kwargs)
            self.set_element_class_lookup(HtmlElementClassLookup())

    class XHTMLParser(etree.XMLParser):
        """An XML parser that is configured to return lxml.html Element
        objects.

        Note that this parser is not really XHTML aware unless you let it
        load a DTD that declares the HTML entities.  To do this, make sure
        you have the XHTML DTDs installed in your catalogs, and create the
        parser like this::

            >>> parser = XHTMLParser(load_dtd=True)

        If you additionally want to validate the document, use this::

            >>> parser = XHTMLParser(dtd_validation=True)

        For catalog support, see http://www.xmlsoft.org/catalog.html.
        """
        def __init__(self, **kwargs):
            super().__init__(**kwargs)
            self.set_element_class_lookup(HtmlElementClassLookup())

    def Element(*args, **kw):
        """Create a new HTML Element.

        This can also be used for XHTML documents.
        """
        v = html_parser.makeelement(*args, **kw)
        return v

    html_parser = HTMLParser()
    xhtml_parser = XHTMLParser()