It's also just annoyingly redundant, and breaks searching, completion, etc.:

    @app.cond('is foo')
    def is_foo():

IMO

    @app.cond
    def is_foo():

And 'losing' that you can omit the underscore to make it look a bit like English is much better. Plus then I can jump from its use back to this definition without any DSL-aware tooling (that probably doesn't exist).