`get_nth_subpath` throws exception

[MqCreaple]

Description of bug / unexpected behavior

In file

manim/manim/mobject/types/vectorized_mobject.py

Lines 1769 to 1772 in d4af5b2

	def get_nth_subpath(path_list, n):
	if n >= len(path_list):
	# Create a null path at the very end
	return [path_list[-1][-1]] * nppcc

, the function get_nth_subpath returns an array referencing path_list[-1][-1]. However, the array path_list is potentially empty, in which case the function will raise an error.

Expected behavior

get_nth_subpath should return a valid value even when path_list is empty.

How to reproduce the issue

Code for reproducing the problem

import numpy as np
from manim import *

class TestScene(Scene):
    def construct(self):
        axes = Axes(x_range=[0, 12 * np.pi, np.pi], y_range=[-1, 1, 0.5])
        curve = axes.plot(lambda x: np.sin(x), x_range=[0, 12 * np.pi])
        timer = ValueTracker(0)
        point1 = Dot(axes.coords_to_point(0, 0, 0))
        point1.add_updater(lambda mob: mob.move_to(axes.coords_to_point(timer.get_value(), np.sin(timer.get_value()), 0.0)))
        lines1 = axes.get_lines_to_point(point1.get_center())
        lines1.add_updater(lambda mob: mob.become(axes.get_lines_to_point(point1.get_center())))

        self.add(axes, curve, point1)
        self.add(lines1)
        self.play(timer.animate.set_value(12 * np.pi), run_time = 5.0)

Additional media files

Images/GIFs

A working version of the previous code should give the following animation:

Logs

Terminal output

[03/07/26 15:10:57] INFO     Animation 0 : Using cached data (hash : 1584795214_3743890524_2038886506)                                                                                                 cairo_renderer.py:94
╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\cli\render\commands.py:1 │
│ 25 in render                                                                                     │
│                                                                                                  │
│   122 │   │   │   try:                                                                           │
│   123 │   │   │   │   with tempconfig({}):                                                       │
│   124 │   │   │   │   │   scene = SceneClass()                                                   │
│ ❱ 125 │   │   │   │   │   scene.render()                                                         │
│   126 │   │   │   except Exception:                                                              │
│   127 │   │   │   │   error_console.print_exception()                                            │
│   128 │   │   │   │   sys.exit(1)                                                                │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\scene\scene.py:259 in    │
│ render                                                                                           │
│                                                                                                  │
│    256 │   │   """                                                                               │
│    257 │   │   self.setup()                                                                      │
│    258 │   │   try:                                                                              │
│ ❱  259 │   │   │   self.construct()                                                              │
│    260 │   │   except EndSceneEarlyException:                                                    │
│    261 │   │   │   pass                                                                          │
│    262 │   │   except RerunSceneException:                                                       │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\test.py:16 in construct                                │
│                                                                                                  │
│   13 │   │                                                                                       │
│   14 │   │   self.add(axes, curve, point1)                                                       │
│   15 │   │   self.add(lines1)                                                                    │
│ ❱ 16 │   │   self.play(timer.animate.set_value(12 * np.pi), run_time = 5.0)                      │
│   17                                                                                             │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\scene\scene.py:1177 in   │
│ play                                                                                             │
│                                                                                                  │
│   1174 │   │   │   return                                                                        │
│   1175 │   │                                                                                     │
│   1176 │   │   start_time = self.time                                                            │
│ ❱ 1177 │   │   self.renderer.play(self, *args, **kwargs)                                         │
│   1178 │   │   run_time = self.time - start_time                                                 │
│   1179 │   │   if subcaption:                                                                    │
│   1180 │   │   │   if subcaption_duration is None:                                               │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\renderer\cairo_renderer. │
│ py:120 in play                                                                                   │
│                                                                                                  │
│   117 │   │   │   # In this case, as there is only a wait, it will be the length of the wait.    │
│   118 │   │   │   self.freeze_current_frame(scene.duration)                                      │
│   119 │   │   else:                                                                              │
│ ❱ 120 │   │   │   scene.play_internal()                                                          │
│   121 │   │   self.file_writer.end_animation(not self.skip_animations)                           │
│   122 │   │                                                                                      │
│   123 │   │   self.num_plays += 1                                                                │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\scene\scene.py:1352 in   │
│ play_internal                                                                                    │
│                                                                                                  │
│   1349 │   │   │   self.duration,                                                                │
│   1350 │   │   )                                                                                 │
│   1351 │   │   for t in self.time_progression:                                                   │
│ ❱ 1352 │   │   │   self.update_to_time(t)                                                        │
│   1353 │   │   │   if not skip_rendering and not self.skip_animation_preview:                    │
│   1354 │   │   │   │   self.renderer.render(self, t, self.moving_mobjects)                       │
│   1355 │   │   │   if self.stop_condition is not None and self.stop_condition():                 │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\scene\scene.py:1678 in   │
│ update_to_time                                                                                   │
│                                                                                                  │
│   1675 │   │   │   animation.update_mobjects(dt)                                                 │
│   1676 │   │   │   alpha = t / animation.run_time                                                │
│   1677 │   │   │   animation.interpolate(alpha)                                                  │
│ ❱ 1678 │   │   self.update_mobjects(dt)                                                          │
│   1679 │   │   self.update_meshes(dt)                                                            │
│   1680 │   │   self.update_self(dt)                                                              │
│   1681                                                                                           │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\scene\scene.py:381 in    │
│ update_mobjects                                                                                  │
│                                                                                                  │
│    378 │   │   │   Change in time between updates. Defaults (mostly) to 1/frames_per_second      │
│    379 │   │   """                                                                               │
│    380 │   │   for mobj in self.mobjects:                                                        │
│ ❱  381 │   │   │   mobj.update(dt)                                                               │
│    382 │                                                                                         │
│    383 │   def update_meshes(self, dt: float) -> None:                                           │
│    384 │   │   for obj in self.meshes:                                                           │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\mobject\mobject.py:901   │
│ in update                                                                                        │
│                                                                                                  │
│    898 │   │   │   │   if "dt" in inspect.signature(updater).parameters:                         │
│    899 │   │   │   │   │   updater(self, dt)                                                     │
│    900 │   │   │   │   else:                                                                     │
│ ❱  901 │   │   │   │   │   updater(self)                                                         │
│    902 │   │   if recursive:                                                                     │
│    903 │   │   │   for submob in self.submobjects:                                               │
│    904 │   │   │   │   submob.update(dt, recursive=recursive)                                    │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\test.py:12 in <lambda>                                 │
│                                                                                                  │
│    9 │   │   point1 = Dot(axes.coords_to_point(0, 0, 0))                                         │
│   10 │   │   point1.add_updater(lambda mob: mob.move_to(axes.coords_to_point(timer.get_value(    │
│   11 │   │   lines1 = axes.get_lines_to_point(point1.get_center())                               │
│ ❱ 12 │   │   lines1.add_updater(lambda mob: mob.become(axes.get_lines_to_point(point1.get_cen    │
│   13 │   │                                                                                       │
│   14 │   │   self.add(axes, curve, point1)                                                       │
│   15 │   │   self.add(lines1)                                                                    │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\mobject\mobject.py:3171  │
│ in become                                                                                        │
│                                                                                                  │
│   3168 │   │   │   if match_center:                                                              │
│   3169 │   │   │   │   mobject.move_to(self.get_center())                                        │
│   3170 │   │                                                                                     │
│ ❱ 3171 │   │   self.align_data(mobject, skip_point_alignment=True)                               │
│   3172 │   │   for sm1, sm2 in zip(self.get_family(), mobject.get_family(), strict=False):       │
│   3173 │   │   │   sm1.points = np.array(sm2.points)                                             │
│   3174 │   │   │   sm1.interpolate_color(sm1, sm2, 1)                                            │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\mobject\mobject.py:2891  │
│ in align_data                                                                                    │
│                                                                                                  │
│   2888 │   │   │   self.align_points(mobject)                                                    │
│   2889 │   │   # Recurse                                                                         │
│   2890 │   │   for m1, m2 in zip(self.submobjects, mobject.submobjects, strict=False):           │
│ ❱ 2891 │   │   │   m1.align_data(m2)                                                             │
│   2892 │                                                                                         │
│   2893 │   def get_point_mobject(self, center=None):                                             │
│   2894 │   │   """The simplest :class:`~.Mobject` to be transformed to or from self.             │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\mobject\mobject.py:2891  │
│ in align_data                                                                                    │
│                                                                                                  │
│   2888 │   │   │   self.align_points(mobject)                                                    │
│   2889 │   │   # Recurse                                                                         │
│   2890 │   │   for m1, m2 in zip(self.submobjects, mobject.submobjects, strict=False):           │
│ ❱ 2891 │   │   │   m1.align_data(m2)                                                             │
│   2892 │                                                                                         │
│   2893 │   def get_point_mobject(self, center=None):                                             │
│   2894 │   │   """The simplest :class:`~.Mobject` to be transformed to or from self.             │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\mobject\mobject.py:2888  │
│ in align_data                                                                                    │
│                                                                                                  │
│   2885 │   │   self.null_point_align(mobject)                                                    │
│   2886 │   │   self.align_submobjects(mobject)                                                   │
│   2887 │   │   if not skip_point_alignment:                                                      │
│ ❱ 2888 │   │   │   self.align_points(mobject)                                                    │
│   2889 │   │   # Recurse                                                                         │
│   2890 │   │   for m1, m2 in zip(self.submobjects, mobject.submobjects, strict=False):           │
│   2891 │   │   │   m1.align_data(m2)                                                             │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\mobject\types\vectorized │
│ _mobject.py:1786 in align_points                                                                 │
│                                                                                                  │
│   1783 │   │                                                                                     │
│   1784 │   │   for n in range(n_subpaths):                                                       │
│   1785 │   │   │   # For each pair of subpaths, add points until they are the same length        │
│ ❱ 1786 │   │   │   sp1 = get_nth_subpath(subpaths1, n)                                           │
│   1787 │   │   │   sp2 = get_nth_subpath(subpaths2, n)                                           │
│   1788 │   │   │   diff1 = max(0, (len(sp2) - len(sp1)) // nppcc)                                │
│   1789 │   │   │   diff2 = max(0, (len(sp1) - len(sp2)) // nppcc)                                │
│                                                                                                  │
│ C:\Users\creaple\Documents\projects\manim\.venv\Lib\site-packages\manim\mobject\types\vectorized │
│ _mobject.py:1772 in get_nth_subpath                                                              │
│                                                                                                  │
│   1769 │   │   def get_nth_subpath(path_list, n):                                                │
│   1770 │   │   │   if n >= len(path_list):                                                       │
│   1771 │   │   │   │   # Create a null path at the very end                                      │
│ ❱ 1772 │   │   │   │   return [path_list[-1][-1]] * nppcc                                        │
│   1773 │   │   │   path = path_list[n]                                                           │
│   1774 │   │   │   # Check for useless points at the end of the path and remove them             │
│   1775 │   │   │   # https://github.com/ManimCommunity/manim/issues/1959                         │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
IndexError: list index out of range

System specifications

System Details

• OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)):
• RAM:
• Python version (python/py/python3 --version):
• Installed modules (provide output from pip list):

PASTE HERE

LaTeX details

• LaTeX distribution (e.g. TeX Live 2020):
• Installed LaTeX packages:

Additional comments

[clawdney]

Hi! I'm interested in helping with this. Could you share more context about the use case?

[MqCreaple]

@clawdney Thanks for your kind support. This issue basically occurred when I tried to animate a point moving along a sine curve, with horizontal and vertical dashed lines to indicate the point's position. The "Additional media files" in my issue description provide the effect I want to achieve with my code.

However, I believe that my pull request #4630 already solves this issue. There is not much left to do besides verifying that the fix is correct. The PR only changes 3 lines, and I'm quite certain that it does not have unintended behaviours.