SelectionList -> OptionList.render_line Unhandled IndexError

[ddkasa]

The Bug

When using a SelectionList the render_line method of the new option list is throwing an IndexError exception when the strips are empty.

Traceback

│                                                                                                                                       │
│ /home/dk/dev/tmp/textual-sandbox/.venv/lib/python3.12/site-packages/textual/widgets/_selection_list.py:514 in render_line             │
│                                                                                                                                       │
│   511 │   │   # TODO: This is rather crufty and hard to fathom. Candidate for a rewrite.       ╭──────── locals ────────╮             │
│   512 │   │                                                                                    │ self = SelectionList() │             │
│   513 │   │   # First off, get the underlying prompt from OptionList.                          │    y = 0               │             │
│ ❱ 514 │   │   line = super().render_line(y)                                                    ╰────────────────────────╯             │
│   515 │   │                                                                                                                           │
│   516 │   │   # We know the prompt we're going to display, what we're going to do                                                     │
│   517 │   │   # is place a CheckBox-a-like button next to it. So to start with                                                        │
│                                                                                                                                       │
│ /home/dk/dev/tmp/textual-sandbox/.venv/lib/python3.12/site-packages/textual/widgets/_option_list.py:860 in render_line                │
│                                                                                                                                       │
│   857 │   │                                                                                                                           │
│   858 │   │   strips = self._get_option_render(option, style)                                                                         │
│   859 │   │                                                                                                                           │
│ ❱ 860 │   │   return strips[line_offset]                                                                                              │
│   861 │                                                                                                                               │
│   862 │                                                                                                                               │
│   863 │   def validate_highlighted(self, highlighted: int | None) -> int | None:                                                      │
│                                                                                                                                       │
│ ╭──────────────────────────────────────── locals ─────────────────────────────────────────╮                                           │
│ │ component_class = 'option-list--option-highlighted'                                     │                                           │
│ │     line_number = 0                                                                     │                                           │
│ │     line_offset = 0                                                                     │                                           │
│ │      mouse_over = False                                                                 │                                           │
│ │          option = Selection(<text 'Professional nor' [] ''>)                            │                                           │
│ │    option_index = 0                                                                     │                                           │
│ │            self = SelectionList()                                                       │                                           │
│ │          strips = []                                                                    │                                           │
│ │           style = <Style background=Color(81, 73, 115) foreground=Color(169, 177, 214)> │                                           │
│ │               y = 0                                                                     │                                           │
│ ╰─────────────────────────────────────────────────────────────────────────────────────────╯                                           │
╰───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
IndexError: list index out of range

MRE

from textual.app import App
from textual.widgets import SelectionList


class BugReportApp(App):
    def compose(self):
        yield SelectionList(("Professional nor", 1))


if __name__ == "__main__":
    BugReportApp().run()

I found this error extremely flakey at times, as sometimes it would only pop up in test runs, so let me know if there is additional info I can provide.

Looking through the previous OptionList, I found that it had an IndexError exception handler at this point, so maybe that should be reverted?

textual/src/textual/widgets/_option_list.py

Lines 884 to 887 in 1c23d6c

	try:
	strip = strips[y_offset]
	except IndexError:
	return Strip([])

Sidenote is that the strips are being first nulled at the RichVisual.render_strips method here:

textual/src/textual/visual.py

Lines 305 to 314 in 699cc86

	strips = [
	Strip(line)
	for line in islice(
	Segment.split_and_crop_lines(
	segments, width, include_new_lines=False, pad=False
	),
	None,
	height,
	)
	]

Textual Diagnostics

Versions

Name	Value
Textual	2.1.0
Rich	13.9.4

Python

Name	Value
Version	3.12.5
Implementation	CPython
Compiler	GCC 14.2.1 20240801 (Red Hat 14.2.1-1)
Executable	/home/dk/dev/tmp/textual-sandbox/.venv/bin/python

Operating System

Name	Value
System	Linux
Release	6.12.13-100.fc40.x86_64
Version	#1 SMP PREEMPT_DYNAMIC Sat Feb 8 17:10:01 UTC 2025

Terminal

Name	Value
Terminal Application	Kitty
TERM	xterm-kitty
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=137, height=54
legacy_windows	False
min_width	1
max_width	137
is_terminal	True
encoding	utf-8
max_height	54
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

[github-actions]

We found the following entry in the FAQ which you may find helpful:

• Does Textual support images?

Feel free to close this issue if you found an answer in the FAQ. Otherwise, please give us a little time to review.

This is an automated reply, generated by FAQtory

[TomJGooding]

Funnily enough I just hit a similar IndexError, but with the OptionList itself while trying to create a MRE for another possible bug.

In my case, the exception occurred after trying to resize my terminal to full screen. I can't reproduce it every time but fairly reliably after resizing the terminal for a while.

[ddkasa]

I was trying to the reproduce it the way you described, but it doesn't want to error. I am curious what terminal emulator are you using?

[TomJGooding]

I don't think this is related to specific emulators. Try messing around with your terminal size with this app:

from textual.app import App, ComposeResult
from textual.widgets import OptionList


class ExampleApp(App):
    def compose(self) -> ComposeResult:
        yield OptionList(
            *[
                f"This is option {i} which is long and may wrap when resized"
                for i in range(100)
            ]
        )


if __name__ == "__main__":
    app = ExampleApp()
    app.run()

[ddkasa]

Yeah, I managed to reproduce the same error with a lot resizing with the example you gave.

[github-actions]

Don't forget to star the repository!

Follow @textualizeio for Textual updates.