Simple Python HTML template DSL

html.py

__all__ = []

def export(value):
    __all__.append(value.__name__)
    return value

import sys
module = sys.modules[__name__]

class Context(object):
    level = 0
    indent = '  '

    def __enter__(self):
        self.level += 1
        return self.__lshift__

    def __exit__(self, type_, value, btrace):
        self.level -= 1

    def __lshift__(self, data):
        print(self.indent * self.level + str(data))
        return self

class StrContext(Context):
    data = ''

    def __lshift__(self, data):
        self.data += self.indent * self.level + str(data) + "\n"
        return self

    def __str__(self):
        return self.data

context = StrContext()
__all__.append('context')

def build_attrs(attrs):
    return ' ' + ' '.join(
            '%s="%s"' % (
                name.replace("_", "-").strip('-'),
                (lambda v: ' '.join(v) if hasattr(v, '__iter__') else v)(
                    (value() if hasattr(value, '__call__') else value)
                ))
                for (name, value) in attrs.iteritems() if value is not None
            ) if attrs else ''

@export
def css(**attrs):
    return ' '.join("%s: %s;" % (name.replace('_', '-'), value)
            for (name, value) in attrs.iteritems())

class TAG(object):
    name = ''
    attrs = {}

    class __metaclass__(type):
        def __enter__(cls):
            return cls().__enter__()

        def __exit__(cls, type_, value, btrace):
            context.level -= 1
            context << '</%s>' % cls.name

        def __getattr__(cls, name):
            return cls[name.replace('_', '-')]

        def __getitem__(cls, name):
            self = cls()
            self.attrs['class'] = [name]
            return self

    def __enter__(self):
        context << '<%s%s>' % (self.name, build_attrs(self.attrs))
        context.level += 1
        return context

    def __exit__(self, type_, value, btrace):
        context.level -= 1
        context << '</%s>' % self.name

    def __call__(self, _content=None, **attrs):
        if attrs:
            self.attrs.update(attrs)

        if _content is not None:
            context << '<%s%s>%s</%s>' % (self.name, build_attrs(self.attrs), _content, self.name)

        return self

    def __init__(self, _content=None, **attrs):
        self.attrs = self.attrs.copy()
        self(_content, **attrs)

    def __getattr__(self, name):
        return self[name.replace('_', '-')]

    def __getitem__(self, name):
        self.attrs.setdefault('class', []).append(name)
        return self

class EMPTYTAG(object):
    name = ''
    attrs = {}

    def __new__(cls, **attrs):
        if attrs:
            _attrs = cls.attrs.copy()
            _attrs.update(attrs)
        else:
            _attrs = cls.attrs

        context << '<%s%s />' % (cls.name, build_attrs(_attrs))

@export
class COMMENT(TAG):
    def __enter__(self):
        context << '<!-- '
        return context

    def __exit__(self, type_, value, traceback):
        context << ' -->'

@export
class HTML(TAG):
    name = 'html'
    doctype = ''

    def __enter__(self):
        context << '''<!DOCTYPE %s>''' % (self.doctype)
        return super(HTML, self).__enter__()

@export
class HTML5(HTML):
    doctype = 'HTML'

simple_tags = ('head body title ' + # Main elements
            'div p blockquote ' + # Blocks
            'h1 h2 h3 h4 h5 h6 ' + # Headers
            'u b i s a em strong span font ' + # Inline markup
            'del ins ' + # Annotation
            'ul ol li dd dt dl ' + # Lists
            'article section nav aside ' + # HTML5
            'audio video object embed param ' + # Media
            'fieldset legend button textarea label select option ' + # Forms
            'table thead tbody tr th td caption ' + # Tables
            ''
            )
empty_tags = 'meta link br hr input'

for tag in simple_tags.split():
    name = tag.upper()
    __all__.append(name)
    setattr(module, name, type(name, (TAG,), {'name': tag}))

for tag in empty_tags.split():
    name = tag.upper()
    __all__.append(name)
    setattr(module, name, type(name, (EMPTYTAG,), {'name': tag}))

@export
class SCRIPT(TAG):
    name = 'script'
    attrs = {'type': 'text/javascript'}

@export
class CSS(LINK):
    attrs = {'type': 'text/css', 'rel': 'stylesheet'}

    def __new__(cls, href):
        super(CSS, cls).__new__(cls, href=href)

@export
class JS(SCRIPT):
    def __init__(self, src):
        super(JS, self).__init__('', src=src)

@export
class FORM(TAG):
    name = 'form'
    attrs = {'method': 'POST'}

    def __init__(self, action='', **attrs):
        super(FORM, self).__init__(action=action, **attrs)

Example markup:

from html import *

def layout():
    def head(title, styles=[], scripts=[], meta={}, http={}, prop={}):
        with HEAD:
            TITLE(title)

            for style in styles:
                CSS(style)

            for script in scripts:
                JS(script)

            for name, value in meta:
                META(name=name, content=value)

            for name, value in http:
                META(http_equiv=name, content=value)

            for name, value in prop:
                META(property=name, content=value)

    def field(id, name=None, value=None, label=None, type='text', help=None):
        with DIV.control_group:
            if label is not None:
                LABEL.control_label(label, for_=id)

            with DIV.controls:
                INPUT(id=id, type=type, name=name, value=value)
                if help is not None:
                    P.help_block(help)

    with HTML5:
        head('Page title',
                styles=['http://netdna.bootstrapcdn.com/twitter-bootstrap/2.0.4/css/bootstrap-combined.min.css'],
                scripts=['http://code.jquery.com/jquery.min.js',
                    'http://netdna.bootstrapcdn.com/twitter-bootstrap/2.0.4/js/bootstrap.min.js'])

        with BODY.main:
            with DIV.content(id='content'):
                H1('Hello, world!')

                with FORM('/my-page').well.form_horizontal, FIELDSET:
                    field(id='input01', label='Text input', help='This is a help text.')
                    field(id='input02', label='Checkbox', type='checkbox')

                    with DIV.control_group:
                        LABEL.control_label('Select list', for_='input03')

                        with DIV.controls, SELECT(id='input03'):
                            for option in xrange(1, 9):
                                OPTION(option)

                    with DIV.form_actions:
                        BUTTON(type='submit').btn.btn_primary('Save changes')
                        BUTTON.btn('Cancel')

layout()
print(context)

Output:

<!DOCTYPE HTML>
<html>
  <head>
    <title>Page title</title>
    <link href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.0.4/css/bootstrap-combined.min.css" type="text/css" rel="stylesheet" />
    <script src="http://code.jquery.com/jquery.min.js" type="text/javascript"></script>
    <script src="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.0.4/js/bootstrap.min.js" type="text/javascript"></script>
  </head>
  <body class="main">
    <div class="content" id="content">
      <h1>Hello, world!</h1>
      <form action="/my-page" method="POST" class="well form-horizontal">
        <fieldset>
          <div class="control-group">
            <label for="input01" class="control-label">Text input</label>
            <div class="controls">
              <input type="text" id="input01" />
              <p class="help-block">This is a help text.</p>
            </div>
          </div>
          <div class="control-group">
            <label for="input02" class="control-label">Checkbox</label>
            <div class="controls">
              <input type="checkbox" id="input02" />
            </div>
          </div>
          <div class="control-group">
            <label for="input03" class="control-label">Select list</label>
            <div class="controls">
              <select id="input03">
                <option>1</option>
                <option>2</option>
                <option>3</option>
                <option>4</option>
                <option>5</option>
                <option>6</option>
                <option>7</option>
                <option>8</option>
              </select>
            </div>
          </div>
          <div class="form-actions">
            <button type="submit" class="btn btn-primary">Save changes</button>
            <button class="btn">Cancel</button>
          </div>
        </fieldset>
      </form>
    </div>
  </body>
</html>

Hi! I've mode some corrections to improve thread-safe support.
Here is the link - https://gist.github.com/kostyll/4e76a79f93e4c807fdd1
If you have a bit of free time, I would be glad you to look through my code :)