How to assign a material to a single face of a mesh using Python API

Question

I have tried the following but the mat is not assigned to any of the faces.

import bpy
import bmesh

# Creating a new Mat
mat = bpy.data.materials.new(name="newMat")
mat.diffuse_color = (0.5,0,0.5,1)        

# Generating a Box
bpy.ops.mesh.primitive_cube_add(size=2,  enter_editmode=True, location=(0, 0, 0))

# Resizing and Translating the box
bpy.ops.mesh.select_all(action='SELECT')
bpy.ops.transform.resize(value=(0.5, 1, 1))
bpy.ops.transform.translate(value=(0.5, 0, 0))
bpy.ops.mesh.select_all(action='DESELECT')

# Instantiating the bmesh of the Box
obj = bpy.context.edit_object
me = obj.data
bm = bmesh.from_edit_mesh(me)

# Selecting just one of the faces
bm.faces[0].select = True      


# toggle to edit mode
bpy.ops.object.mode_set(mode='EDIT')

# make sure face select mode is enabled
bpy.context.tool_settings.mesh_select_mode = [False, False, True]


# use second material slot
ob = bpy.context.active_object
ob.active_material_index = 1


# assign the material
bpy.ops.object.material_slot_assign()

# toggle to object mode
bpy.ops.object.mode_set(mode='OBJECT')

What am I doing wrong, or how else could I achieve this?

Answer 1

You can assign the materials using MeshPolygon.material_index in Object Mode without Bmesh.

import bpy

def create_material(mat_name, diffuse_color=(1,1,1,1)):
    mat = bpy.data.materials.new(name=mat_name)
    mat.diffuse_color = diffuse_color
    return mat

# Generate 2 demo materials
mat_red = create_material("Red", (1,0,0,1))
mat_green = create_material("Green", (0,1,0,1))

# Generating a Box
bpy.ops.mesh.primitive_cube_add(size=2, location=(0, 0, 0))
# Resizing and Translating the box ...

# Get a reference to the object in context
obj = bpy.context.object

# Append both Materials to the created object
obj.data.materials.append(mat_red)
obj.data.materials.append(mat_green)

# Assign the green Material to the second polygon 
obj.data.polygons[1].material_index = 1

# Assign the green Material to the 4th polygon 
obj.data.polygons[3].material_index = 1

Related:

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• select specific face by its index

Answer 2

If you'd like to assign material(s) to certain faces in Edit Mode, just assign the index of the material slot to BMFace.material_index on a Bmesh representation of your mesh:

import bpy, bmesh

def create_material(mat_name, diffuse_color=(1,1,1,1)):
    mat = bpy.data.materials.new(name=mat_name)
    mat.diffuse_color = diffuse_color
    return mat

# Generate 2 demo materials
mat_red = create_material("Red", (1,0,0,1))
mat_green = create_material("Green", (0,1,0,1))

# Generate a Box
bpy.ops.mesh.primitive_cube_add(size=2, enter_editmode=True, location=(0, 0, 0))
# Resize and Translate the box ...

# Get a reference to the object in context
obj = bpy.context.object

# Append both Materials to the created object
obj.data.materials.append(mat_red)
obj.data.materials.append(mat_green)

# Bmesh representation
me = obj.data
bm = bmesh.from_edit_mesh(me)
bm.faces.ensure_lookup_table()

# Assign the green Material to the second polygon
bm.faces[1].material_index = 1

# Assign the green Material to the 4th polygon 
bm.faces[3].material_index = 1

# Optional
#bmesh.update_edit_mesh(me)
#bpy.ops.object.mode_set(mode='OBJECT')

You might want to create the mesh using Bmesh in this case as well: how to use BMesh to add verts, faces, and edges to existing geometry