Format python data structures in HTML no dependencies

My poorly written tests suggests that the module works as intended. I fear this is to good to be true. I don't known how to write unit tests for this. I'm new to writing tests and I think this module would be hard for a unit test master to test.

#-*-coding:utf8;-*-
#qpy:3
#qpy:console


SINGLETONS = [
    'area',
    'base',
    'br',
    'col',
    'command',
    'embed',
    'hr',
    'img',
    'input',
    'keygen',
    'link',
    'meta',
    'param',
    'source',
    'track',
    'wbr'
]

# Constants.
SP = ' '
EMPTY = ''


def is_singleton(tag):
    return tag in SINGLETONS

def not_singleton(tag):
    return is_singleton(tag) == False

def html_attributes(**kw):
    # 'attrs' is the elements attributes.
    # Iterate over the keys and values of the kw dict
    # and transform them into a string of html 
    # attributes. Two html attribute keys are 
    # Python keywords 'class' and 'id' to set
    # the id and class of and element use:
    # cls for class and '_id' for 'id.'
    attrs = EMPTY
    n_attrs = len(kw)
    for key, value in zip(kw.keys(), kw.values()):
        if key == 'cls':
            key = 'class'

        if key == '_id':
            key = 'id'

        if n_attrs > 1:
            attrs += '{}="{}"{}'.format(key, value, SP)
        else:
           attrs += '{}="{}"'.format(key, value)
    return attrs.rstrip(SP)


def tagify(tagname, data=EMPTY, **kw):
    if isinstance(data, str):
        data = data.replace('\n', '<br>')
    attrs = html_attributes(**kw)

    if not attrs:
        opentag = '<{}>'.format(tagname)
    else:
        opentag = '<{}{}{}>'.format(tagname, SP, attrs)

    if not_singleton(tagname):
        closetag = '</{}>'.format(tagname)
    else:
        closetag = None

    if not closetag:
        return '{}'.format(opentag)

    if data:
        return '{}{}{}'.format(opentag, data, closetag)
    else:
        return '{}{}'.format(opentag, closetag)


def tag(tagname, **deco_kw):
    '''
    Decorate a functions output with html by
    passing it through tagify.
    '''
    def deco(func):
        def wraps(*args, **kw):
            content = func(*args, **kw)
            return tagify(tagname, content, **deco_kw)
        return wraps
    return deco


def tests():
    '''
    This is a temporary function for
    testing the module.
    Please dont include this in any reviews.
    '''
    @tag('li', cls='link', _id='list-item')
    def link(name, **kw):
        return tagify('a', name, **kw)

    @tag('article', cls='main', _id='spam')
    def paragraph(content, **kw):
        return tagify('p', content, **kw)

    print(link(__name__, src=__file__))
    print(paragraph(list(range(10)), _id='monty'))


if __name__ == '__main__':
    tests()

test() output

<li id="list-item" class="link"><a src="/">__main__</a></li>
<article id="spam" class="main"><p id="monty">[0, 1, 2, 3, 4]</p></article>