code: def execute(self, callback=None):
          
          if callback is None:
              for _ in self:
                  pass
          elif not callable(callback):
              raise TypeError('callback must be callable, not ' +
                              repr(callback))
          else:
              for instance in self:
                  callback(instance)
docs: Execute the plan.  If optional ``callback`` is present,
          it is invoked with an :class:`~sqlalchemy_imageattach.entity.Image`
          instance for every migrated image.
  
          :param callback: an optional callback that takes
                           an :class:`~sqlalchemy_imageattach.entity.Image`
                           instance.  it's called zero or more times
          :type callback: :class:`~typing.Callable`\ [[:class:`~.entity.Image`],
                                                       :const:`None`]