shape_key_from_group.py

import bpy
from bpy.types import Operator, DATA_PT_shape_keys

bl_info = {
    "name": "Shape key mirror",
    "description": "",
    "author": "Lateasusual",
    "version": (1, 0, 0),
    "blender": (2, 81, 0),
    "location": "Properties > Mesh > Shape Keys",
    "warning": "",
    "wiki_url": "",
    "tracker_url": "",
    "support": "COMMUNITY",
    "category": "Mesh"
}


def mirror(name: str):
    if ".L" in name:
        return name.replace(".L", ".R", 1)
    if ".R" in name:
        return name.replace(".R", ".L", 1)
    return None


def is_mirror(name: str):
    if ".L" in name:
        return "L"
    if ".R" in name:
        return "R"
    return None


class MESH_OT_MirrorShapeGroup(Operator):
    bl_idname = 'mesh.mirror_shape_key_group'
    bl_label = 'Make mirror from group'
    bl_description = "Create a mirrored shape key with mirrored vertex group"
    bl_options = {'REGISTER', 'UNDO'}

    @classmethod
    def poll(cls, context):
        obj = context.object
        return obj and obj.type in {'MESH', 'LATTICE'}

    def execute(self, context):
        ob = context.object
        keys = ob.data.shape_keys
        kb = ob.active_shape_key
        groups = ob.vertex_groups
        vg1 = kb.vertex_group

        # Unset everything
        kb.vertex_group = ""
        vals = [k.value for k in keys.key_blocks]
        for k in keys.key_blocks:
            k.value = 0.0
        kb.value = kb.slider_max

        vg2 = mirror(vg1)
        if vg2 is None:
            self.report({'ERROR'}, f"Vertex group {vg1} must be mirrorable (.L or .R)")
            return {'CANCELLED'}

        if vg2 not in groups:
            self.report({'ERROR'}, f"No mirror group '{vg2}' found!")
            return {'CANCELLED'}

        mir = is_mirror(kb.name)

        if mir is None:
            kb.name += "." + is_mirror(vg1)

        kb2 = ob.shape_key_add(name=mirror(kb.name), from_mix=True)
        kb.vertex_group = vg1
        kb2.vertex_group = vg2

        for (i, v) in enumerate(vals):
            keys.key_blocks[i].value = v

        return {'FINISHED'}


def draw(self, context):
    layout: bpy.types.UILayout = self.layout

    ob = context.object
    key = ob.data.shape_keys
    kb = ob.active_shape_key

    if kb:
        if key.use_relative and ob.active_shape_key_index != 0:
            if mirror(ob.active_shape_key.vertex_group) is not None:
                layout.use_property_split = True
                layout.operator('mesh.mirror_shape_key_group')


def register():
    bpy.utils.register_class(MESH_OT_MirrorShapeGroup)
    DATA_PT_shape_keys.append(draw)


def unregister():
    bpy.utils.unregister_class(MESH_OT_MirrorShapeGroup)
    DATA_PT_shape_keys.remove(draw)


if __name__ == "__main__":
    register()