Blender 2.9 Python GPU Module, drawing lines in the wrong position

Question

I have problems with the GPU Module and rendering custom geometry in Blender. When I run the example code from the Blender docs, it works as expected. Running the first line drawing example produces the image below, drawing a thin red line ten units up in Z:

import bpy
import gpu
from gpu_extras.batch import batch_for_shader

coords = [(0, 0, 0), (0, 0, 10)]
shader = gpu.shader.from_builtin('3D_UNIFORM_COLOR')
batch = batch_for_shader(shader, 'LINES', {"pos": coords})


def draw():
    shader.bind()
    shader.uniform_float("color", (1, 0, 0, 1))
    batch.draw(shader)


bpy.types.SpaceView3D.draw_handler_add(draw, (), 'WINDOW', 'POST_VIEW')

But, when I try to encapsulate the code in a simple class, things get weird. The same coordinates in the code produces a line in a weird position, downwards at an angle instead of straight up. And I have to execute the code twice to make it even show up.

import bpy
import gpu
from gpu_extras.batch import batch_for_shader


class drawManager:
    
    coords = []
    shader = None
    batch = None
    handle = None

    def __init__(self):
        self.coords = [(0, 0, 0), (0, 0, 10)]
        self.shader = gpu.shader.from_builtin('3D_UNIFORM_COLOR')
        self.batch = batch_for_shader(self.shader, 'LINES', {"pos": self.coords})
        

    def draw(self):
        self.shader.bind()
        self.shader.uniform_float("color", (1, 0, 0, 1))
        self.batch.draw(self.shader)

    def doDraw(self):
        handle = bpy.types.SpaceView3D.draw_handler_add(self.draw, (), 'WINDOW', 'POST_VIEW')
        
    def stopDraw(self):
        bpy.types.SpaceView3D.draw_handler_remove(self.handle, 'WINDOW')
        

dm = drawManager()
dm.draw()
dm.doDraw()

Am I missing something essential here or is this a weird bug?

Answer 1

The problem seems to arise only when using uniform_float(). Instead I put the position/color information inside the batch_for_shader()-function and update it whenever necessary. It now works like a charm.