Demonstration of using Python to take an existing mesh, selecting a particular face, and extruding it

select_face_in_blender_and_extrude.py

import bpy,bmesh

# Assumes we aleady have a cube out, nothing selected
ob = bpy.data.objects['Cube']
bpy.ops.object.mode_set(mode='EDIT')
mesh = bmesh.from_edit_mesh(bpy.context.object.data)
ur_face = None

# look at all of the faces of the cube, find the one that is 'facing' the positive direction on the y axis
for f in mesh.faces:
    if f.normal.y == 1.0:
        f.select = True
        face_to_extrude = f
        
# command to make an inplace extrusion
extruded_face = bmesh.ops.extrude_face_region(mesh, geom=[face_to_extrude])

# translate the newly extruded face along the green y axis in 3D space
bmesh.ops.translate(mesh, vec=(0,2,0), verts=extruded_face['geom'][-1].verts)

# trigger viewport update
bpy.context.scene.objects.active = bpy.context.scene.objects.active