executing modifier on object does not have any effect

Question

I have a dataset, called ModelNet, containing *.off-files. I need to read each file, decimate the mesh and store it to another file.

Problem: For some reason the modifier I add to the model has no effect on the mesh.

What I did: On Blender 2.79b I have included an add-on to read off files. It works. Then I have set the object I loaded from the *.off-file to active.

Note that I have added to break keywords, since I tried to make the decimation work for one mesh, until I let it loop through every file.

I have tried the code of this post before. Unfortunately I was not able to get that code work for me.

Now, the output of number of vertices, edges and polygons does not change through decimation.

Question: What am I doing wrong? I would appreciate help!

Code:

numberOfIteration=2
decimateRatio=0.3
modifierName='DecimateMod'

# Cleans all decimate modifiers
def cleanAllDecimateModifiers(obj):
    for m in obj.modifiers:
        if(m.type=="DECIMATE"):
            obj.modifiers.remove(modifier=m)

# remove the mesh cube (the intial one from startup)
if "Cube" in bpy.data.meshes:
    mesh = bpy.data.meshes["Cube"]
    print("removing mesh", mesh)
    bpy.data.meshes.remove(mesh)

datasetPath = '~/ModelNet10_Mesh/raw'
folders = next(os.walk(datasetPath))[1]
print(folders)
for folder in folders:
    files = next(os.walk(datasetPath + '/' + str(folder) + '/test/'))[2]

    for file in files:

        bpy.ops.import_mesh.off(filepath=datasetPath + '/' + str(folder) + '/test/' + str(file), axis_forward='-Z', axis_up='Y', filter_glob="*.off")

        print(file.split('.')[0])
        bpy.data.objects[file.split('.')[0]].select = True  # works until blender 2.79
        # bpy.data.objects[file.split('.')[0]].select_set(True)  # works for blender 2.8+
        bpy.context.scene.objects.active = bpy.data.objects[file.split('.')[0]]

        for i in range(0, numberOfIteration):
            objectList = bpy.data.objects
            for obj in objectList:
                if(obj.type == "MESH"):
                    print("before decimation")
                    print(obj.data.vertices)
                    print(obj.data.edges)
                    print(obj.data.polygons)

                    cleanAllDecimateModifiers(obj)

                    bpy.ops.object.modifier_add(type='DECIMATE')
                    bpy.context.object.modifiers["Decimate"].use_collapse_triangulate = True
                    bpy.context.object.modifiers["Decimate"].ratio = 1-decimateRatio*(i+1)
                    bpy.context.scene.update()

                    print("after decimation")
                    print(obj.data.vertices)
                    print(obj.data.edges)
                    print(obj.data.polygons)
        break
    break

Console output:

Camera
Cube
Lamp
['Scene']
removing mesh <bpy_struct, Mesh("Cube")>
['bathtub', 'toilet', 'desk', 'night_stand', 'bed', 'dresser', 'sofa', 'monitor', 'chair', 'table']
bathtub_0141
before decimation
<bpy_collection[74637], MeshVertices>
<bpy_collection[75181], MeshEdges>
<bpy_collection[25241], MeshPolygons>
after decimation
<bpy_collection[74637], MeshVertices>
<bpy_collection[75181], MeshEdges>
<bpy_collection[25241], MeshPolygons>
before decimation
<bpy_collection[74637], MeshVertices>
<bpy_collection[75181], MeshEdges>
<bpy_collection[25241], MeshPolygons>
after decimation
<bpy_collection[74637], MeshVertices>
<bpy_collection[75181], MeshEdges>
<bpy_collection[25241], MeshPolygons>

EDIT: According to @batFINGER I have now used the following code:

# remove the mesh cube (the intial one from startup)
if "Cube" in bpy.data.meshes:
    mesh = bpy.data.meshes["Cube"]
    print("removing mesh", mesh)
    bpy.data.meshes.remove(mesh)

context = bpy.context
scene = context.scene

datasetPath = '~/datasets/ModelNet10_Mesh/raw'
folder = 'bed'
file = 'bed_0534.off'
bpy.ops.import_mesh.off(filepath=datasetPath + '/' + str(folder) + '/test/' + str(file), axis_forward='-Z', axis_up='Y', filter_glob="*.off")

mesh_obs = [o for o in scene.objects if o.type == "MESH"]

def delMods(ob, mod_type="ALL"):
    mods = (ob.modifiers[:] if mod_type == "ALL"
            else [m for m in ob.modifiers if m.type == mod_type])
    while(mods):
        ob.modifiers.remove(mods.pop())

for ob in mesh_obs:
    delMods(ob, 'DECIMATE')
    dm = ob.modifiers.new("Xxx", type="DECIMATE")
    dm.use_collapse_triangulate = True
    me = ob.data
    me_mod = ob.to_mesh(scene, True, 'RENDER')
    # print(len(me.vertices), len(me_mod.vertices))
    print("me - vertices, polygons, edges")
    print(me.vertices)
    print(me.polygons)
    print(me.edges)
    print("me_mod - vertices, polygons, edges")
    print(me_mod.vertices)
    print(me_mod.polygons)
    print(me_mod.edges)

which results in the following console output:

removing mesh <bpy_struct, Mesh("Cube")>
me - vertices, polygons, edges
<bpy_collection[54904], MeshVertices>
<bpy_collection[30636], MeshPolygons>
<bpy_collection[73232], MeshEdges>
me_mod - vertices, polygons, edges
<bpy_collection[54904], MeshVertices>
<bpy_collection[30636], MeshPolygons>
<bpy_collection[73232], MeshEdges>

scene structure:

Answer 1

Object.to_mesh

In 2.79 can use the to mesh method.

>>> me = C.object.to_mesh(
to_mesh()
Object.to_mesh(scene, apply_modifiers, settings, calc_tessface=True, calc_undeformed=False)
Create a Mesh data-block with modifiers applied
>>> me = C.object.to_mesh()

To create a mesh with the modifiers applied, instead of using modifier apply operator. Will apply all modifiers, not just the decimate modifier.

Note for 2.8 this has changed see How do I get a mesh data-block with modifiers and shape keys applied in Blender 2.8?

Test script, creates a modified mesh to export. Prints the vert count of original and mesh returned from Object.to_mesh(...) Looped thru all mesh objects in scene. It is often the case that after an import the objects created are the only ones selected and can be garnished from context.selected_objects as opposed to using the dataset file structure.

import bpy

context = bpy.context
scene = context.scene

mesh_obs = [o for o in scene.objects if o.type=='MESH']
# if after op call the newly imported objects are only ones selected use
#mesh_obs = [o for o in context.selected_objects if o.type=='MESH']    
def delmods(ob, mod_type="ALL"):
    mods = (ob.modifiers[:] if mod_type == "ALL" 
            else [m for m in ob.modifiers if m.type == mod_type])
    while(mods):
        ob.modifiers.remove(mods.pop())

for ob in mesh_obs:
    delmods(ob, 'DECIMATE')
    dm = ob.modifiers.new("Xxx", type='DECIMATE')
    dm.ratio = 0.5 # change to suit
    dm.use_collapse_triangulate = True
    me = ob.data
    me_mod = ob.to_mesh(scene, True, 'RENDER')
    print(len(me.vertices), len(me_mod.vertices))
    #bpy.data.meshes.remove(me_mod) #  clean up

Result of running on default cube.

me - vertices, polygons, edges
<bpy_collection[8], MeshVertices>
<bpy_collection[6], MeshPolygons>
<bpy_collection[12], MeshEdges>
me_mod - vertices, polygons, edges
<bpy_collection[5], MeshVertices>
<bpy_collection[6], MeshPolygons>
<bpy_collection[9], MeshEdges>