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An example of a simple addon which adds an add spindle option to the add mesh menu. (Wow, lots of ads in that one, pardon the pun.)


bl_info = {
    "name": "Spindle",
    "author": "Your_name_here, Your_co_developers_name_here",
    "version": (0, 1),
    "blender": (2, 55, 0),
    "location": "View3D > Add > Mesh",
    "description": "Adds Spindle Object.",
    "warning": "",
    "wiki_url": "http://wiki.blender.org/index.php/Extensions:2.5/Py/"\
        "Scripts/Add_Mesh/",
    "tracker_url": "",
    "category": "Add Mesh"}
 
import bpy
from mathutils import *
from math import *
from bpy.props import *
 
 
def align_matrix(context):
    loc = Matrix.Translation(context.scene.cursor_location)
    obj_align = context.user_preferences.edit.object_align
    if (context.space_data.type == 'VIEW_3D'
        and obj_align == 'VIEW'):
        rot = context.space_data.region_3d.view_matrix.rotation_part().invert().resize4x4()
    else:
        rot = Matrix()
    align_matrix = loc * rot
    return align_matrix
 
 
def create_mesh_object(context, verts, edges, faces, name, edit, align_matrix):
    scene = context.scene
    obj_act = scene.objects.active
 
    # Can't edit anything, unless we have an active obj.
    if edit and not obj_act:
        return None
 
    # Create new mesh
    mesh = bpy.data.meshes.new(name)
 
    # Make a mesh from a list of verts/edges/faces.
    mesh.from_pydata(verts, edges, faces)
 
    # Update mesh geometry after adding stuff.
    mesh.update()
 
    # Deselect all objects.
    bpy.ops.object.select_all(action='DESELECT')
 
    if edit:
        # Replace geometry of existing object
 
        # Use the active obj and select it.
        ob_new = obj_act
        ob_new.select = True
 
        if obj_act.mode == 'OBJECT':
            # Get existing mesh datablock.
            old_mesh = ob_new.data
 
            # Set object data to nothing
            ob_new.data = None
 
            # Clear users of existing mesh datablock.
            old_mesh.user_clear()
 
            # Remove old mesh datablock if no users are left.
            if (old_mesh.users == 0):
                bpy.data.meshes.remove(old_mesh)
 
            # Assign new mesh datablock.
            ob_new.data = mesh
 
    else:
        # Create new object
        ob_new = bpy.data.objects.new(name, mesh)
 
        # Link new object to the given scene and select it.
        scene.objects.link(ob_new)
        ob_new.select = True
 
        # Place the object at the 3D cursor location.
        # apply viewRotaion
        ob_new.matrix_world = align_matrix
 
    if obj_act and obj_act.mode == 'EDIT':
        if not edit:
            # We are in EditMode, switch to ObjectMode.
            bpy.ops.object.mode_set(mode='OBJECT')
 
            # Select the active object as well.
            obj_act.select = True
 
            # Apply location of new object.
            scene.update()
 
            # Join new object into the active.
            bpy.ops.object.join()
 
            # Switching back to EditMode.
            bpy.ops.object.mode_set(mode='EDIT')
 
            ob_new = obj_act
 
    else:
        # We are in ObjectMode.
        # Make the new object the active one.
        scene.objects.active = ob_new
 
    return ob_new
 
 
def createFaces(vertIdx1, vertIdx2, closed=False, flipped=False):
    faces = []
 
    if not vertIdx1 or not vertIdx2:
        return None
 
    if len(vertIdx1) < 2 and len(vertIdx2) < 2:
        return None
 
    fan = False
    if (len(vertIdx1) != len(vertIdx2)):
        if (len(vertIdx1) == 1 and len(vertIdx2) > 1):
            fan = True
        else:
            return None
 
    total = len(vertIdx2)
 
    if closed:
        # Bridge the start with the end.
        if flipped:
            face = [
                vertIdx1[0],
                vertIdx2[0],
                vertIdx2[total - 1]]
            if not fan:
                face.append(vertIdx1[total - 1])
            faces.append(face)
 
        else:
            face = [vertIdx2[0], vertIdx1[0]]
            if not fan:
                face.append(vertIdx1[total - 1])
            face.append(vertIdx2[total - 1])
            faces.append(face)
 
    # Bridge the rest of the faces.
    for num in range(total - 1):
        if flipped:
            if fan:
                face = [vertIdx2[num], vertIdx1[0], vertIdx2[num + 1]]
            else:
                face = [vertIdx2[num], vertIdx1[num],
                    vertIdx1[num + 1], vertIdx2[num + 1]]
            faces.append(face)
        else:
            if fan:
                face = [vertIdx1[0], vertIdx2[num], vertIdx2[num + 1]]
            else:
                face = [vertIdx1[num], vertIdx2[num],
                    vertIdx2[num + 1], vertIdx1[num + 1]]
            faces.append(face)
 
    return faces
 
 
def add_spindle(segments, radius, height, cap_height):
    verts = []
    faces = []
 
    tot_verts = segments * 2 + 2
 
    half_height = height / 2.0
 
    # Upper tip
    idx_upper_tip = len(verts)
    verts.append(Vector((0, 0, half_height + cap_height)))
 
    # Lower tip
    idx_lower_tip = len(verts)
    verts.append(Vector((0.0, 0.0, -half_height - cap_height)))
 
    upper_edgeloop = []
    lower_edgeloop = []
    for index in range(segments):
        mtx = Matrix.Rotation(2.0 * pi * float(index) / segments, 3, 'Z')
 
        # Calculate index & location of upper verte4x tip.
        idx_up = len(verts)
        upper_edgeloop.append(idx_up)
        verts.append(Vector((radius, 0.0, half_height)) * mtx)
 
        if height > 0:
            idx_low = len(verts)
            lower_edgeloop.append(idx_low)
            verts.append(Vector((radius, 0.0, -half_height)) * mtx)
 
    # Create faces for the upper tip.
    tip_up_faces = createFaces([idx_upper_tip], upper_edgeloop,
        closed=True, flipped=True)
    faces.extend(tip_up_faces)
 
    if height > 0:
        # Create faces for the middle cylinder.
        cyl_faces = createFaces(lower_edgeloop, upper_edgeloop, closed=True)
        faces.extend(cyl_faces)
 
        # Create faces for the lower tip.
        tip_low_faces = createFaces([idx_lower_tip], lower_edgeloop,
            closed=True)
        faces.extend(tip_low_faces)
 
    else:
        # Skipping middle part/cylinder (height=0).
 
        # Create faces for the lower tip.
        tip_low_faces = createFaces([idx_lower_tip], upper_edgeloop,
            closed=True)
        faces.extend(tip_low_faces)
 
    return verts, faces
 
class AddSpindle(bpy.types.Operator):
    '''Add a spindle mesh.'''
    bl_idname = "mesh.primitive_spindle_add"
    bl_label = "Add Spindle"
    bl_description = "Create a spindle mesh."
    bl_options = {'REGISTER', 'UNDO'}
 
    # edit - Whether to add or update.
    edit = BoolProperty(name="",
        description="",
        default=False,
        options={'HIDDEN'})
    segments = IntProperty(name="Segments",
        description="Number of segments of the spindle",
        min=3,
        max=512,
        default=32)
    radius = FloatProperty(name="Radius",
        description="Radius of the spindle",
        min=0.01,
        max=9999.0,
        default=1.0)
    height = FloatProperty(name="Height",
        description="Height of the spindle",
        min=0.0,
        max=100.0,
        default=1.0)
    cap_height = FloatProperty(name="Cap Height",
        description="Cap height of the spindle",
        min=-9999.0,
        max=9999.0,
        default=0.5)
    align_matrix = Matrix()
 
    def execute(self, context):
 
        verts, faces = add_spindle(
            self.segments,
            self.radius,
            self.height,
            self.cap_height)
 
        obj = create_mesh_object(context, verts, [], faces, "Spindle",
            self.edit, self.align_matrix)
 
        return {'FINISHED'}
 
    def invoke(self, context, event):
        self.align_matrix = align_matrix(context)
        self.execute(context)
        return {'FINISHED'}
 
def menu_func(self, context):
    self.layout.operator(AddSpindle.bl_idname, text="Spindle", icon='PLUGIN')
 
 
def register():
    bpy.utils.register_module(__name__)
 
    bpy.types.INFO_MT_mesh_add.append(menu_func)
 
 
def unregister():
    bpy.utils.unregister_module(__name__)
 
    bpy.types.INFO_MT_mesh_add.remove(menu_func)
 
if __name__ == "__main__":
    register()