parameters: (self, fn)
docstring: Attaches a callable that will be called when the future finishes.
  
          Args:
              fn: A callable that will be called with this future as its only
                  argument when the future completes or is cancelled. The callable
                  will always be called by a thread in the same process in which
                  it was added. If the future has already completed or been
                  cancelled then the callable will be called immediately. These
                  callables are called in the order that they were added.
docstring_summary: Attaches a callable that will be called when the future finishes.
docstring_tokens: Attaches a callable that will be called when the future finishes .
function: def add_done_callback(self, fn):
          """Attaches a callable that will be called when the future finishes.
  
          Args:
              fn: A callable that will be called with this future as its only
                  argument when the future completes or is cancelled. The callable
                  will always be called by a thread in the same process in which
                  it was added. If the future has already completed or been
                  cancelled then the callable will be called immediately. These
                  callables are called in the order that they were added.
          """
          with self._condition:
              if self._state not in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED]:
                  self._done_callbacks.append(fn)
                  return
          fn(self)
function_tokens: def add_done_callback ( self , fn ) : with self . _condition : if self . _state not in [ CANCELLED , CANCELLED_AND_NOTIFIED , FINISHED ] : self . _done_callbacks . append ( fn ) return fn ( self )