Type-ahead completion

A how-to guide: a field with the best few matches beneath it, updating at each keystroke, chosen by a digit.

from collections.abc import Sequence

from sextile import Lines, MenuItem, OnOneFrame, Page, PageAddress, PageLayout, PageRequest, PageRouter, Sextile
from sextile.forms import TypeAhead

router = PageRouter()

_PLACES = ["Trondheim", "Tromsø", "Troon", "Truro", "Oslo", "Bergen"]

@router.page("3", name="search", title="Find a place")
async def search(request: PageRequest) -> Page:
    async def lookup(typed: str) -> Sequence[MenuItem]:
        matches = [name for name in _PLACES if name.upper().startswith(typed.upper())]
        return [MenuItem(text=name, detail="", destination=PageAddress("31")) for name in matches[:3]]

    field = TypeAhead(lookup=lookup, label="PLACE:", no_match="No place of that name.")
    return PageLayout(
        parts=[OnOneFrame(Lines(("Key a place name.", ""))), OnOneFrame(field)],
    ).build(request)

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

 FIND A PLACE                         3a
  
Key a place name.                       
                                        
 PLACE:   TRO                           
                                        
 1  Trondheim  #                        
 2  Tromso                              
 3  Troon                               
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
  
 0 index                                

lookup is awaited with what has been keyed and returns the entries to suggest; letters type into the field, a digit chooses the numbered match — leading wherever that entry’s destination says — and RETURN takes the one marked with #. no_match is said when nothing matches.

Why three suggestions and not nine: the list is redrawn on the wire at each keystroke, and nine rows cost nearly three seconds at 1200 baud where three cost about one. The measurement is in Viewdata encoding, the reasoning in Design decisions. The form API is sextile.forms; for a plain field the reader types into, see Accept typed input.

What the reader has typed so far is held for the connection — the session drives the form between keystrokes, so the terminal keeps nothing. Session state in general is Remember a caller.