Configuring an App's media_type makes it set the said type on all responses unless specified otherwise.

However, the default HTTP error serializer doesn't know how to serialize HTML. So it tries to negotiate an accepted media type that it has a handler for; supported options include JSON, XML (deprecated), and any other media types you have configured response media handlers for.

You can mainly customize this in two different ways.

Setting a custom error serializer.
Implementing a response media handler for falcon.MEDIA_HTML.

For instance, (2) could look like:

import falcon
import falcon.asgi
import falcon.media

HTML_ERROR_TEMPLATE = """
<html>
<head><title>{title}</title></head>
<body>
<center><h1>{title}</h1></center><hr>
{attrs}
</body>
</html>
"""


class HTMLHandler(falcon.media.BaseHandler):
    def serialize(self, media: object, content_type: str) -> bytes:
        assert isinstance(media, dict), 'TODO: How to serialize other types?'
        data = media.copy()
        title = data.pop('title', 'Data')
        attrs = '\n'.join(f'<center><b>{key}</b>: {value}<center>'
                          for key, value in data.items())
        return HTML_ERROR_TEMPLATE.format(title=title, attrs=attrs).encode()


class QuoteResource:
    async def on_get(self, req: falcon.asgi.Request, resp: falcon.asgi.Response) -> None:
        """Handle GET requests."""
        resp.media = {
            'quote': "I've always been more interested in the future than in the past.",
            'author': 'Grace Hopper',
        }


app = falcon.asgi.App(media_type=falcon.MEDIA_HTML)
app.resp_options.media_handlers[falcon.MEDIA_HTML] = HTMLHandler()
app.add_route('/quote', QuoteResource())

You can now render http://localhost:8000/ and http://localhost:8000/quote, etc.

If you want to render custom HTML error templates, and don't intend to serialize data as HTML, (1) is probably a better option. Adapting our previous example:

import falcon
import falcon.asgi
import falcon.media

HTML_ERROR_TEMPLATE = """
<html>
<head><title>{title}</title></head>
<body>
<center><h1>{title}</h1></center><hr>
{attrs}
</body>
</html>
"""


def serialize_http_error(
    req: falcon.asgi.Request, resp: falcon.asgi.Response, exception: falcon.HTTPError
) -> None:
    data = exception.to_dict()
    title = data.pop('title', 'Error')
    attrs = '\n'.join(f'<center><b>{key}</b>: {value}<center>'
                      for key, value in data.items())
    resp.content_type = falcon.MEDIA_HTML
    resp.text = HTML_ERROR_TEMPLATE.format(title=title, attrs=attrs)


class QuoteResource:
    async def on_get(self, req: falcon.asgi.Request, resp: falcon.asgi.Response) -> None:
        """Handle GET requests."""
        resp.content_type = falcon.MEDIA_JSON
        resp.media = {
            'quote': "I've always been more interested in the future than in the past.",
            'author': 'Grace Hopper',
        }


app = falcon.asgi.App()
app.set_error_serializer(serialize_http_error)
app.add_route('/quote', QuoteResource())

Falcon doesn't do this by default because it is primarily API-oriented, so media types are typically JSON, YAML, Msgpack, etc.

Uh oh!

Media type HTML error message #2596

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