How to tell which function is causing errors from bpy.app.handlers?

Question

Some add-ons can add their own handlers which may cause errors or even crash.

How to tell which functions are causing problems?

Asking on behalf of this report.

Answer 1

You can write functions which intercept the existing functions and print out information, eg:

Given the following test example:

import bpy
def test_handler_a(scene):
    print("  Some Pre Handler", scene)
bpy.app.handlers.frame_change_pre.append(test_handler_a)

def test_handler_b(scene):
    print("  Some Post Handler", scene)
bpy.app.handlers.frame_change_post.append(test_handler_b)

This script sets up intercepting functions that print out all handlers that run:

import bpy
import sys

def intercept(fn, *args):
    print("Intercepting:", fn)
    sys.stdout.flush()

    fn(*args)

    print("... done")
    sys.stdout.flush()

for attr in dir(bpy.app.handlers):
    if attr.startswith("_"):
        continue

    handler_list = getattr(bpy.app.handlers, attr)
    if not isinstance(handler_list, list):
        continue
    if not handler_list:
        continue

    print("Intercept Setup:", attr)

    handler_list[:] = [lambda *args: intercept(fn, *args) for fn in handler_list]