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Function from Python not being loaded for Driver when project opened
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System Information
Operating system: Pop!_OS (Debian) 18.10
Graphics card: GTX Titan X

Blender Version
2.80 77eaa4790d37
Worked: (optional)

A driver calls on a python function. The python function is internal and registered.

When the project is opened, the driver can't find the function, throwing up "ERROR: Invalid Python expression".

Pressing "Update Dependencies" fixes the problem

Exact steps for others to reproduce the error
With 'driver-bug-test.blend':

  • Open the project.
  • Enter pose mode for the armature
  • Try moving it along the X axis (Moving the armature along the x axis should make the cube move along the Y-axis, but it doesn't)
  • Exit pose mode
  • Select the cube
  • In the Drivers panel, you'll see in the Driver Property panel an error "ERROR: Invalid Python expression"
  • Clicking 'Update Dependencies' allows the driver to start working correctly.

This happens each time the project is opened.

I'd be happy to just click this button each time I load, but if you have a project with many items being driven by functions, it's not practical at all.

Event Timeline

Brecht Van Lommel (brecht) lowered the priority of this task from 90 to 50.Jan 31 2019, 6:34 PM

@Sergey Sharybin (sergey), this traces back to rBd192d723123a: Ensure dependency graph exists before initializing editors.

Python module registration should be done before the depsgraph like in 2.7 I think, for cases like this. Wouldn't mind an extra pair of eyes to check if it's safe though.

diff --git a/source/blender/windowmanager/intern/wm_files.c b/source/blender/windowmanager/intern/wm_files.c
index 120bf65..5295064 100644
--- a/source/blender/windowmanager/intern/wm_files.c
+++ b/source/blender/windowmanager/intern/wm_files.c
@@ -491,12 +491,6 @@ static void wm_file_read_post(
        CTX_wm_window_set(C, wm->windows.first);
-       Main *bmain = CTX_data_main(C);
-       DEG_on_visible_update(bmain, true);
-       wm_event_do_depsgraph(C);
-       ED_editors_init(C);
        if (is_startup_file) {
                /* possible python hasn't been initialized */
@@ -526,6 +520,12 @@ static void wm_file_read_post(
        UNUSED_VARS(is_startup_file, reset_app_template);
 #endif  /* WITH_PYTHON */
+       Main *bmain = CTX_data_main(C);
+       DEG_on_visible_update(bmain, true);
+       wm_event_do_depsgraph(C);
+       ED_editors_init(C);
        /* important to do before NULL'ing the context */