parameters: (self, fn)
docstring: Called by receiver to trigger a call fn(error) when an error
          arrives. If an error result is already set, fn(error) is called
          immediately.
docstring_summary: Called by receiver to trigger a call fn(error) when an error
          arrives. If an error result is already set, fn(error) is called
          immediately.
docstring_tokens: Called by receiver to trigger a call fn ( error ) when an error arrives . If an error result is already set fn ( error ) is called immediately .
function: def setErrback(self, fn):
          """Called by receiver to trigger a call fn(error) when an error
          arrives. If an error result is already set, fn(error) is called
          immediately.
          """
          assert callable(fn), "fn nees to be a callable object"
          with self._lock:  # locking needed in case the errback is set, then the sender sets the error, then _errback is called twice
              self._errback = fn
              if self._read and self._error:
                  self._errback(self._error)
function_tokens: def setErrback ( self , fn ) : assert callable ( fn ) , "fn nees to be a callable object" with self . _lock : # locking needed in case the errback is set, then the sender sets the error, then _errback is called twice self . _errback = fn if self . _read and self . _error : self . _errback ( self . _error )