Why there is no option to add custom url converters to blueprints like for main app?

Question

In this post and in official docs we saw how to add custom url converters for main app object. Here is short example:

app = Flask(__name__)
app.url_map.converters['list'] = ListConverter

But how to do it for blueprints? This global (app level) custom converter is unavailable for blueprints. In source code I haven't found such posibility...

Answer 1

The technical reason why you can't have custom URL converters on a blueprint is that unlike applications, blueprints do not have a URL map.

When you use the blueprint's route decorator or add_url_map() method all the blueprint does is record the intention to call the application versions of these methods later when register_blueprint() is called.

I'm not sure there is a benefit in allowing blueprint specific url converters. But I think it would be reasonable to allow a blueprint to install an app wide converter. That could use the same techniques as other blueprint app-wide handlers, like before_app_request, for example.

def add_app_url_converter(self, name, f):
    self.record_once(lambda s: s.app.url_map.converters[name] = f
    return f

Blueprint.add_app_url_converter = add_app_url_converter

# ...

bp = Blueprint('mybp', __name__)
bp.add_app_url_converter('list', ListConverter)