code: def url_value_preprocessor(self, fn):
          
          self._defer(lambda app: app.url_value_preprocessor(fn))
          return fn
docs: Register a URL value preprocessor function for all view
          functions in the application. These functions will be called before the
          :meth:`before_request` functions.
  
          The function can modify the values captured from the matched url before
          they are passed to the view. For example, this can be used to pop a
          common language code value and place it in ``g`` rather than pass it to
          every view.
  
          The function is passed the endpoint name and values dict. The return
          value is ignored.