How to change object geometry over time in the same model from the command line?

Question

I'd like to write a python script can change (not delete and re-create) the geometry of an object AFTER the initial object has already been loaded by blender.

To clarify, here are the 3 basic steps:

1. I write a python file (import bpy, time, ...) that includes some geometric object.
2. Through the command line, I open blender and import the python file into blender.

~/tools/Blender-2.79-CellBlender/blender -P ~/src4/.../filename.py

3. blender opens, the initial geometry loads, then the geometry periodically changes as the python file runs through a time delayed function that changes the geometry of that object.

I don't want blender to open a new window every time the geometry changes, it should just update the existing model.

I believe this is called periodic display, but I couldn't find any helpful resources online.

I'm running Debian 8, Blender 2.79, python 3.7.3.

In case it's relevent, here's my python code that I'm importing in:

import bpy, time

#remove all default objects
bpy.data.objects['Cube'].select = True
bpy.data.objects['Camera'].select = True
bpy.data.objects['Lamp'].select = True
bpy.ops.object.delete() 

#time delayed loop that adds geometry. I want to CHANGE, not add the geometry. 
for x in range(1,10):
    len = x
    #Define box
    verts = [(-len,.5,.3),
             (len,.5,.3),
             (len,-.5,.3),
             (-len,-.5,.3),
             (-len,.5,-.3),
             (len,.5,-.3),
             (len,-.5,-.3),
             (-len,-.5,-.3),
            ]
    faces = [(0, 1, 2, 3),(4,5,6,7),(0,4,7,3),(1,5,6,2),(0,1,5,4),(3,2,6,7)]
    edges = []

    my_mesh = bpy.data.meshes.new('Meshy')
    my_object = bpy.data.objects.new('Objey', my_mesh)
    bpy.context.scene.objects.link(my_object)
    my_mesh.update(calc_edges=True)
    time.sleep(1)

Right now it just waits 10 seconds to run through the loop, then opens blender with 9 separate boxes, as one would expect. As indicated above, this is not what I want to do.

Answer 1

Well, I still can't find a way to change an object, but I did find out how to delete the current object and create a slightly altered object for every frame.

The key is to use bpy.app.handlers:

import bpy, math

#remove all default objects
bpy.data.objects['Cube'].select = True
bpy.data.objects['Camera'].select = True
bpy.data.objects['Lamp'].select = True
bpy.ops.object.delete() 

#Define box
verts = [(0,0,0),
         (0,0,0),
         (0,0,0),
         (0,0,0),
         (0,0,0),
         (0,0,0),
         (0,0,0),
         (0,0,0),
         ]
faces = [(0, 1, 2, 3),(4,5,6,7),(0,4,7,3),(1,5,6,2),(0,1,5,4),(3,2,6,7)]
edges = []

my_mesh = bpy.data.meshes.new('Meshy')
my_object = bpy.data.objects.new('Objey', my_mesh)
bpy.context.scene.objects.link(my_object)

my_mesh.from_pydata(verts, edges, faces)
my_mesh.update(calc_edges=True)

def change_geom(scene):
    #assign frame number
    f = bpy.context.scene.frame_current
    #assign vertex values
    d = 5*math.sin(f/1000*180/3.141592)
    z = 1+math.sin(f/(250)*180/3.141592)

    #delete previous geometry
    bpy.data.objects['Objey'].select = True
    bpy.ops.object.delete() 

    #create new geometry
    verts = [(-d,d,z),
             (d,d,z),
             (d,-d,z),
             (-d,-d,z),
             (-d,d,-z),
             (d,d,-z),
             (d,-d,-z),
             (-d,-d,-z),
            ]
    faces = [(0, 1, 2, 3),(4,5,6,7),(0,4,7,3),(1,5,6,2),(0,1,5,4),(3,2,6,7)]
    edges = []

    my_mesh = bpy.data.meshes.new('Meshy')
    my_object = bpy.data.objects.new('Objey', my_mesh)
    bpy.context.scene.objects.link(my_object)

    my_mesh.from_pydata(verts, edges, faces)

#calls the function every time you change frames
bpy.app.handlers.frame_change_pre.append(change_geom)

Still looking to find a way to alter existing geometry instead of re-creating it (which is way less efficient)!