Calling imgui.get_draw_data() gives segfault about pyimgui

[swistakm]

I have reproduced the issue. I was testing on pygame & windows 10. I was using following snippet:

sel = imgui.selectable('click me to give draw list & segfault', False)
if sel[1]:  # clicked
    print("draws")
    draws = imgui.get_draw_data()
    print(draws)
    print(draws.commands_lists)

In my situation it doesn't break immidiately on imgui.get_draw_data() call but on draws.commands_lists attribute access. @pconerly is it the same for you?

Regarding the possible cause, I have to look deeper into C++ sources to confirm that but I'm pretty confident you should not try to access the draw data and commands lists before they are finalized with imgui.render() call (or Render() in C++). If you already use some built-in integration (Pygame, PySDL2, etc.) it will mean that window is already drawn.

If you really want to access the draw commands before they are actually drawn you'll have to install your own render callback in that will override one installed by PySDL2 integration. This requires some trickery but is definitely doable:

...
window, gl_context = impl_pysdl2_init()
renderer = SDL2Renderer(window)

def overridden_callback(draw_data):
    print(draw_data)
    print(draw_data.commands_lists)
    renderer.render(draw_data)

io = imgui.get_io()
io.render_callback = overridden_callback
...

[swistakm]

I wonder if we could/should make some kind of safeguard around these to make sure we will raise an exception in such condition. This will guard us against segfaults because these in Python are usually hard to debug.

Edit: We may also check if newer versions of ImGui have some additional asserts in such places. If so, they would be automatically translated to Python exceptions.