Application Handlers (bpy.app.handlers)

This module contains callback lists

Basic Handler Example

This script shows the most simple example of adding a handler.

import bpy


def my_handler(scene):
    print("Frame Change", scene.frame_current)


bpy.app.handlers.frame_change_pre.append(my_handler)

Persistent Handler Example

By default handlers are freed when loading new files, in some cases you may want the handler stay running across multiple files (when the handler is part of an add-on for example).

For this the bpy.app.handlers.persistent decorator needs to be used.

import bpy
from bpy.app.handlers import persistent


@persistent
def load_handler(dummy):
    print("Load Handler:", bpy.data.filepath)


bpy.app.handlers.load_post.append(load_handler)

Note on Altering Data

Altering data from handlers should be done carefully. While rendering the frame_change_pre and frame_change_post handlers are called from one thread and the viewport updates from a different thread. If the handler changes data that is accessed by the viewport, this can cause a crash of Blender. In such cases, lock the interface (Render → Lock Interface or bpy.types.RenderSettings.use_lock_interface) before starting a render.

Below is an example of a mesh that is altered from a handler:

def frame_change_pre(scene):
    # A triangle that shifts in the z direction
    zshift = scene.frame_current * 0.1
    vertices = [(-1, -1, zshift), (1, -1, zshift), (0, 1, zshift)]
    triangles = [(0, 1, 2)]

    object = bpy.data.objects["The Object"]
    object.data.clear_geometry()
    object.data.from_pydata(vertices, [], triangles)

bpy.app.handlers.depsgraph_update_post

on depsgraph update (post)

bpy.app.handlers.depsgraph_update_pre

on depsgraph update (pre)

bpy.app.handlers.frame_change_post

Called after frame change for playback and rendering, after the data has been evaluated for the new frame.

bpy.app.handlers.frame_change_pre

Called after frame change for playback and rendering, before any data is evaluated for the new frame. This makes it possible to change data and relations (for example swap an object to another mesh) for the new frame. Note that this handler is not to be used as ‘before the frame changes’ event. The dependency graph is not available in this handler, as data and relations may have been altered and the dependency graph has not yet been updated for that.

bpy.app.handlers.load_factory_preferences_post

on loading factory preferences (after)

bpy.app.handlers.load_factory_startup_post

on loading factory startup (after)

bpy.app.handlers.load_post

on loading a new blend file (after)

bpy.app.handlers.load_pre

on loading a new blend file (before)

bpy.app.handlers.redo_post

on loading a redo step (after)

bpy.app.handlers.redo_pre

on loading a redo step (before)

bpy.app.handlers.render_cancel

on canceling a render job

bpy.app.handlers.render_complete

on completion of render job

bpy.app.handlers.render_init

on initialization of a render job

bpy.app.handlers.render_post

on render (after)

bpy.app.handlers.render_pre

on render (before)

bpy.app.handlers.render_stats

on printing render statistics

bpy.app.handlers.render_write

on writing a render frame (directly after the frame is written)

bpy.app.handlers.save_post

on saving a blend file (after)

bpy.app.handlers.save_pre

on saving a blend file (before)

bpy.app.handlers.undo_post

on loading an undo step (after)

bpy.app.handlers.undo_pre

on loading an undo step (before)

bpy.app.handlers.version_update

on ending the versioning code

bpy.app.handlers.xr_session_start_pre

on starting an xr session (before)

bpy.app.handlers.persistent

Function decorator for callback functions not to be removed when loading new files