Why doesn't the window update in curses? - python-3.x

Question

I took this nice example of a simple curses application with a list. I wanted to make it scrollable, so I changed the part of the list that gets shown. However, I can scroll down and back up, but the contents shown doesn't change (only the highlighted line, not the lines shown).
What am I doing wrong?
MVCE
#!/usr/bin/env python
import curses
from curses import panel
class Menu(object):
def __init__(self, items, stdscreen):
self.window = stdscreen.subwin(0, 0)
self.window.keypad(1)
self.panel = panel.new_panel(self.window)
self.panel.hide()
panel.update_panels()
self.position = 0
self.items = items
def navigate(self, n):
self.position += n
if self.position < 0:
self.position = 0
elif self.position >= len(self.items):
self.position = len(self.items) - 1
def display(self):
self.panel.top()
self.panel.show()
self.window.clear()
while True:
self.window.refresh()
curses.doupdate()
start = 0
# The next 3 lines seem not to work as intended
while start + (curses.LINES - 1) < self.position:
start += curses.LINES
for index, item in enumerate(self.items[start:curses.LINES - 1], start=start):
if index == self.position:
mode = curses.A_REVERSE
else:
mode = curses.A_NORMAL
msg = "%d. %s" % (index, item[0])
self.window.addstr(1 + index, 1, msg, mode)
key = self.window.getch()
if key in [curses.KEY_ENTER, ord("\n"), curses.KEY_RIGHT]:
self.items[self.position][1]()
elif key == curses.KEY_UP:
self.navigate(-1)
elif key == curses.KEY_DOWN:
self.navigate(1)
elif key == curses.KEY_LEFT:
break
self.window.clear()
self.panel.hide()
panel.update_panels()
curses.doupdate()
class MyApp(object):
def __init__(self, stdscreen):
self.screen = stdscreen
curses.curs_set(0)
submenu_items = [("beep", curses.beep), ("flash", curses.flash)]
submenu = Menu(submenu_items, self.screen)
main_menu_items = [
("beep", curses.beep),
("flash", curses.flash),
("submenu", submenu.display),
]
for i in range(200):
main_menu_items.append((f"flash {i}", curses.flash))
main_menu = Menu(main_menu_items, self.screen)
main_menu.display()
if __name__ == "__main__":
curses.wrapper(MyApp)

Answer 1

Basically that's because you're not updating the upper limit on the slice used in this loop:
for index, item in enumerate(self.items[start:curses.LINES - 1], start=start):
Here's a better version
MVCE
#!/usr/bin/env python
import curses
from curses import panel
class Menu(object):
def __init__(self, items, stdscreen):
self.window = stdscreen.subwin(0, 0)
self.window.keypad(1)
self.panel = panel.new_panel(self.window)
self.panel.hide()
panel.update_panels()
self.position = 0
self.items = items
def navigate(self, n):
self.position += n
if self.position < 0:
self.position = 0
elif self.position >= len(self.items):
self.position = len(self.items) - 1
def display(self):
self.panel.top()
self.panel.show()
self.window.clear()
while True:
start = 0
self.window.clear()
while start + (curses.LINES - 1) < self.position:
start += curses.LINES
myrow = self.position - start
mycol = 0
for index, item in enumerate(self.items[start:start + curses.LINES - 1], start=start):
if index == self.position:
mode = curses.A_REVERSE
else:
mode = curses.A_NORMAL
msg = "%d. %s" % (index, item[0])
self.window.addstr(index - start, 0, msg, mode)
if index == self.position:
(myrow, mycol) = self.window.getyx()
self.window.move(myrow, mycol)
key = self.window.getch()
if key in [curses.KEY_ENTER, ord("\n"), curses.KEY_RIGHT]:
self.items[self.position][1]()
elif key == curses.KEY_UP:
self.navigate(-1)
elif key == curses.KEY_DOWN:
self.navigate(1)
elif key == curses.KEY_LEFT:
break
self.window.clear()
self.panel.hide()
panel.update_panels()
curses.doupdate()
class MyApp(object):
def __init__(self, stdscreen):
self.screen = stdscreen
curses.curs_set(1)
submenu_items = [("beep", curses.beep), ("flash", curses.flash)]
submenu = Menu(submenu_items, self.screen)
main_menu_items = [
("beep", curses.beep),
("flash", curses.flash),
("submenu", submenu.display),
]
for i in range(200):
main_menu_items.append((f"flash {i}", curses.flash))
main_menu = Menu(main_menu_items, self.screen)
main_menu.display()
if __name__ == "__main__":
curses.wrapper(MyApp)