code: def add_post_configure_callback(self, callback, run_once=False):
          
          if run_once:
              self._post_configure_callbacks['single'].append(callback)
          else:
              self._post_configure_callbacks['multiple'].append(callback)
  
          return self
docs: Add a new callback to be run after every call to :meth:`configure`.
  
          Functions run at the end of :meth:`configure` are given the
          application's resulting configuration and the arguments passed to
          :meth:`configure`, in that order. As a note, this first argument will
          be an immutable dictionary.
  
          The return value of all registered callbacks is entirely ignored.
  
          Callbacks are run in the order they are registered, but you should
          never depend on another callback.
  
          .. admonition:: The "Resulting" Configuration
  
              The first argument to the callback is always the "resulting"
              configuration from the call to :meth:`configure`. What this means
              is you will get the Application's FROZEN configuration after the
              call to :meth:`configure` finished. Moreover, this resulting
              configuration will be an
              :class:`~werkzeug.datastructures.ImmutableDict`.
  
              The purpose of a Post Configure callback is not to futher alter the
              configuration, but rather to do lazy initialization for anything
              that absolutely requires the configuration, so any attempt to alter
              the configuration of the app has been made intentionally difficult!
  
          Args:
              callback (function):
                  The function you wish to run after :meth:`configure`. Will
                  receive the application's current configuration as the first
                  arugment, and the same arguments passed to :meth:`configure` as
                  the second.
  
          Keyword Args:
              run_once (bool):
                  Should this callback run every time configure is called? Or
                  just once and be deregistered? Pass ``True`` to only run it
                  once.
  
          Returns:
              fleaker.base.BaseApplication:
                  Returns itself for a fluent interface.