Customise the not-found and failed notices

A how-to guide: give the service its own wording for the page a caller could not reach.

from sextile import Page, PageRequest, PageRouter, Sextile, notice_page

router = PageRouter()


@router.page("1", name="index", title="Directory")
async def index(request: PageRequest) -> Page:
    return notice_page(request, "Key a page number.")


app = Sextile(name="Directory", pages=[*router])


@app.on_not_found
async def unknown(request: PageRequest, target: str) -> Page:
    return notice_page(
        request,
        f"There is no page *{target}#.",
        "",
        "Key *1# for the index.",
        title="NO SUCH PAGE",
    )

 NO SUCH PAGE                         1a
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There is no page *7#.                   
                                        
Key *1# for the index.                  
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
  
 0 index                                

on_not_found(request, target) registers the notice for a page the service has not got; on_failed(request, error) and on_timed_out(request, seconds) do the same for a handler that raised and one that ran too long. Each returns a Page, so it is built with notice_page or any layout. The session shows it over the page the reader was on and leaves them there — keying *7# above draws the notice without moving off page 1 — so it names a way on rather than assuming one.

Why the failed notice is a page of its own, distinct from not-found, and why a raised handler costs its page and not the call, is in Design decisions.