A Markdown extension which adds support for bootstrap alerts and (simple) carousels using pymdownx blocks v9.10b5.

bootstrap_alert.py

from pymdownx.blocks import BlocksExtension
from pymdownx.blocks.block import Block, type_html_attribute_dict, type_html_identifier, type_string_in, \
                                  type_boolean, type_string, , type_multi
import xml.etree.ElementTree as etree
import uuid


class BsAlertBlock(Block):
    NAME = 'alert'
    ARGUMENT = None
    OPTIONS = {
        'type': ['primary', type_string_in([
            'primary', 'secondary', 'success', 'danger', 'warning', 'info', 'light', 'dark'
            ], type_html_identifier)],
        'dismissable': [False, type_boolean],
        'markdown': ['inline', type_string_in(['block', 'inline', 'raw', 'auto'])]
    }

    def on_create(self, parent):
        ''' Create wrapper div '''

        # Define the basic attributes needed to make this a Bootstrap alert
        classes = f'alert alert-{self.options["type"]}'
        if self.options['dismissable']:
            # TODO: Maybe leave `fade show` out and let the user define them?
            classes = f'{classes} alert-dismissible fade show'
        attrib = {
            'class': classes,
            'role': 'alert'
        }
        # Build the containing Element
        alert = etree.SubElement(parent, 'div', attrib=attrib)
        if self.argument:
            # Build the title Element
            title = etree.SubElement(alert, 'h4', attrib={'class': 'alert-heading'})
            title.text = self.argument
        return alert

    def on_markdown(self):
        return self.options['markdown']

    def on_end(self, block):
        ''' Add close button to end of block if dismissable '''
        if self.options['dismissable']:
            attrib = {
                'type':'button',
                'class': 'btn-close',
                'data-bs-dismiss': 'alert',
                'aria-label': 'Close'
            }
            etree.SubElement(block, 'button', attrib=attrib)


class BsCarouselBlock(Block):
    NAME = 'carousel'
    ARGUMENT = False
    OPTIONS = {
        'controls': [True, type_boolean],
        'indicators': [False, type_boolean],
        'fade': [False, type_boolean],
        'autoplay': [False, type_multi(type_boolean, type_string_in(['carousel']))]
    }

    def on_init(self):
        self.inner_element = None

    def on_validate(self, parent):
        if 'id' not in self.options['attrs']:
            self.options['attrs']['id'] = str(uuid.uuid4())
        return True

    def on_create(self, parent):
        attrib = {'class': 'carousel slide'}
        if self.options['fade']:
            # TODO: maybe add some class to the carousel-item? See Bootstrap docs.
            attrib['class'] += ' carousel-fade'
        if self.options['autoplay'] is not False:
            attrib['data-bs-ride'] = str(self.options['autoplay']).lower()
        block = etree.SubElement(parent, 'div', attrib=attrib)
        self.inner_element = etree.SubElement(block, 'div', attrib={'class': 'carousel-inner'})
        if self.options['controls']:
            # Add next/previous controls
            for direction in ['prev', 'next']:
                attrib = {
                    'class': f'carousel-control-{direction}',
                    'type': 'button',
                    'data-bs-target': f'#{self.options["attrs"]["id"]}',
                    'data-bs-slide': direction
                }
                text = 'Previous' if direction == 'prev' else 'Next'
                el = etree.SubElement(block, 'button', attrib=attrib)
                etree.SubElement(el, 'span', attrib={'class': f'carousel-control-{direction}-icon', 'aria-hidden': 'true'})
                etree.SubElement(el, 'span', attrib={'class': 'visually-hidden'}).text = text
        return block

    def on_add(self, block):
        return self.inner_element

    def on_markdown(self):
        return 'inline'

    def on_inline_end(self, block):
        ''' Run after inline parsing is complete. '''
        if self.options['indicators']:
            indicators = etree.Element('div', attrib={'class': 'carousel-indicators'})
            block.insert(0, indicators)
        for i, img in enumerate(self.inner_element.findall('img')):
            self.inner_element.remove(img)
            attrib={'class': 'carousel-item'}
            if i == 0:
                attrib['class'] = f'{attrib["class"]} active'
            slide = etree.SubElement(self.inner_element, 'div', attrib=attrib)
            img.set('class', 'd-block w-100')
            slide.append(img)
            if self.options['indicators']:
                attrib = {
                    'data-bs-target': f'#{self.options["attrs"]["id"]}',
                    'data-bs-slide-to': f'{i}',
                    'aria-label': f'Slide {i+1}'
                }
                if i == 0:
                    attrib['class'] = 'active'
                    attrib['aria-current'] = 'true'
                etree.SubElement(indicators, 'button', attrib=attrib)


class BsBlockExtension(BlocksExtension):

    def extendMarkdownBlocks(self, md, block_mgr):
        block_mgr.register(BsAlertBlock, self.getConfigs())
        block_mgr.register(BsCarouselBlock, self.getConfigs())


def makeExtension(*args, **kwargs):
    """Return extension."""

    return BsBlockExtension(*args, **kwargs)


if __name__ == '__main__':
    import markdown
    from textwrap import dedent
    src = dedent('''
    /// alert
    This is an alert.
    ///

    /// alert | Title
    This alert has a title.
    ///

    /// alert | Warning
        type: warning

    This is a **warning** alert with a title.
    ///

    /// alert
        type: secondary
        attrs: {id: custom, class: p5}
        markdown: block

    This alert has custom attributes defined. We have a custom id and custom padding set.
    The custom padding is set using bootstraps' class `p5` which adds padding to all sides.
    ///

    /// alert
        type: danger
        dismissable: true
        markdown: auto

    You can dismiss me!
    ///

    /// carousel
        #attrs: {id: 'random'}
        indicators: true
        #fade: true
        #autoplay: carousel

    ![img 1](img1.jpg)
    ![img 2](img2.jpg)
    ///
    ''')
    body = markdown.markdown(src, extensions=[BsBlockExtension()])
    template = dedent(f'''
    <!doctype html>
    <html lang="en">
      <head>
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <title>Bootstrap demo</title>
        <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha1/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-GLhlTQ8iRABdZLl6O3oVMWSktQOp6b7In1Zl3/Jr59b6EGGoI1aFkw7cmDA6j6gD" crossorigin="anonymous">
      </head>
      <body>
        <h1>Hello, world!</h1>
        {body}
        <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha1/dist/js/bootstrap.bundle.min.js" integrity="sha384-w76AqPfDkMBDXo30jS1Sgez6pr3x5MlQ1ZAGC+nuZB+EYdgRZgiwxhTBTkF7CXvN" crossorigin="anonymous"></script>
      </body>
    </html>
    ''')
    with open('example.html', 'w') as f:
        f.write(template)
    print(body)

output.txt

<div class="alert alert-primary" role="alert">
<p>This is an alert.</p>
</div>
<div class="alert alert-primary" role="alert">
<h4 class="alert-heading">Title</h4>
<p>This alert has a title.</p>
</div>
<div class="alert alert-warning" role="alert">
<h4 class="alert-heading">Warning</h4>
<p>This is a <strong>warning</strong> alert with a title.</p>
</div>
<div class="alert alert-secondary p5" id="custom" role="alert">
<p>This alert has custom attributes defined. We have a custom id and custom padding set.
The custom padding is set using bootstraps' class <code>p5</code> which adds padding to all sides.</p>
</div>
<div class="alert alert-danger alert-dismissible fade show" role="alert">
<p>You can dismiss me!</p>
<button aria-label="Close" class="btn-close" data-bs-dismiss="alert" type="button"></button></div>
<div class="carousel slide" id="9b81eb23-0ec0-4f2b-8ddd-48a36d92f409">
<div class="carousel-indicators"><button aria-current="true" aria-label="Slide 1" class="active" data-bs-slide-to="0" data-bs-target="#9b81eb23-0ec0-4f2b-8ddd-48a36d92f409"></button><button aria-label="Slide 2" data-bs-slide-to="1" data-bs-target="#9b81eb23-0ec0-4f2b-8ddd-48a36d92f409"></button></div>
<div class="carousel-inner">
<div class="carousel-item active"><img alt="img 1" class="d-block w-100" src="img1.jpg" />
</div>
<div class="carousel-item"><img alt="img 2" class="d-block w-100" src="img2.jpg" /></div>
</div>
<button class="carousel-control-prev" data-bs-slide="prev" data-bs-target="#9b81eb23-0ec0-4f2b-8ddd-48a36d92f409" type="button"><span aria-hidden="true" class="carousel-control-prev-icon"></span><span class="visually-hidden">Previous</span></button><button class="carousel-control-next" data-bs-slide="next" data-bs-target="#9b81eb23-0ec0-4f2b-8ddd-48a36d92f409" type="button"><span aria-hidden="true" class="carousel-control-next-icon"></span><span class="visually-hidden">Next</span></button></div>