How To Run Python Script When Blender Starts

GOAL

To run Python script when Blender starts only once or every time.

Environment

Blender 2.83
Windows 10

Method

I created a simple Python script “Documents\blenderPython\test\spheres.py” to add spheres as below

import bpy
for i in range(3):
    bpy.ops.mesh.primitive_uv_sphere_add(radius=1, location=((i-1)*3, 0, 3))

Only once

Start Blender with command

Start blender in command prompt with an argument -P or –python.
Reference: Python Options in Blender 2.91 Manual

blender -P Documents\blenderPython\test\spheres.py

The following is the result of startup scene.

Every time

Method 1. Create bat file.

Create batch file and start Blender with it. Save the command above as “blender_start.bat” and execute it.

Method 2. Hook the function to handler

To run script every time, use handler to execute functions when blender starts. This method can be used in your Addon.
Reference: Application Handlers (bpy.app.handlers) in Blender2.91 Manual

startup_sphere.py

import bpy
from bpy.app.handlers import persistent

@persistent
def add_sphere(dummy):
    for i in range(3):
        bpy.ops.mesh.primitive_uv_sphere_add(radius=1, location=((i - 1) * 3, 0, 3))

def register():
    bpy.app.handlers.load_post.append(add_sphere)
def unregister():
    pass
if __name__ == "__main__":
    register()

Put the file startup_sphere.py in the user startup directory such as C:\Users\<USER_NAME>\AppData\Roaming\Blender Foundation\Blender\2.83\scripts\startup. The files in the start up directory are executed when blender starts.
Reference: Script Loading in Blender Python API document

The following is the result of startup scene.

Wrong Example

You can’t put the python file that has operations of bpy.ops as below in user startup directory.

import bpy
for i in range(3):
    bpy.ops.mesh.primitive_uv_sphere_add(radius=1, location=((i - 1) * 3, 0, 3))

import bpy
from bpy.app.handlers import persistent

def add_sphere():
    for i in range(3):
        bpy.ops.mesh.primitive_uv_sphere_add(radius=1, location=((i - 1) * 3, 0, 3))

def register():
    add_sphere()
def unregister():
    pass
if __name__ == "__main__":
    register()

The error “AttributeError: ‘_RestrictContext’ object has no attribute ‘view_layer'” will occurs when the addon is activated. Check “[Blender Error] AttributeError: ‘_RestrictContext’ object has no attribute ‘view_layer’” for details.