code:

import bpy

# Creates a cube with an X mirror modifier.
def make_mirrored_cube():
    mesh = bpy.data.meshes.new("MCube mesh")
    obj = bpy.data.objects.new("MCube", mesh)
    col = bpy.context.collection
    col.objects.link(obj)
    bpy.context.view_layer.objects.active = obj

    mesh = bpy.context.object.data
    verts = [(1,-1,-1), (1, 1,-1),
             (1,-1, 1), (1, 1, 1),
             (0,-1,-1), (0, 1,-1),
             (0,-1, 1), (0, 1, 1)]
    faces = [(0, 1, 3, 2), (0, 1, 5, 4), (2, 3, 7, 6),
             (0, 2, 6, 4), (1, 3, 7, 5)]
    mesh.from_pydata(verts, [], faces)
    bpy.ops.object.modifier_add(type="MIRROR")
    bpy.context.object.modifiers["Mirror"].use_clip = True
    obj.select_set(state=True)
    bpy.context.view_layer.objects.active = obj

class MakeMirroredCube(bpy.types.Operator):
    bl_idname = "view3d.make_mirrored_cube"
    bl_label = "Make Mirrored Cube"
    def execute(self, context):
        make_mirrored_cube()
        return {'FINISHED'}

def register():
    bpy.utils.register_class(MakeMirroredCube)

def unregister():
    bpy.utils.unregister_class(MakeMirroredCube)

if __name__ == "__main__":
    register()