What does @defer.inlineCallbacks mean when I don't need yield return a value?

Question

At scrapy.core.engine

ExecutionEngine method start

@defer.inlineCallbacks
def start(self):
    """Start the execution engine"""
    assert not self.running, "Engine already running"
    self.start_time = time()
    yield self.signals.send_catch_log_deferred(signal=signals.engine_started)
    self.running = True
    self._closewait = defer.Deferred()
    yield self._closewait

Why don't use self.signals.send_catch_log_deferred(signal=signals.engine_started) directly　but instead of a yield ?

Answer 1

Why don't use self.signals.send_catch_log_deferred(signal=signals.engine_started) directly　but instead of a yield ?

Because send_catch_log_deferred returns a Deferred object. If you want to avoid a yield there, then you should use send_catch_log but the point of using send_catch_log_deferred is to allow listeners to return Deferred objects.

The signals that use send_catch_log are not able to return Deferred objects, hence they do not allow to perform asynchronous operations.

Edit: For a good introduction to inlineCallbacks see: http://krondo.com/?p=2441

Answer 2

@defer.inlineCallbacks expects the decorated function to be a generator function and calling a generator function inside decorated function (even returning one) doesn't make the function, a generator function. An investigation:

def gen():
    yield 1

def func(): return gen

import dis

dis.dis(gen)
  2           0 LOAD_CONST               1 (1)
              3 YIELD_VALUE         
              4 POP_TOP             
              5 LOAD_CONST               0 (None)
              8 RETURN_VALUE        

dis.dis(func)
  1           0 LOAD_GLOBAL              0 (gen)
              3 RETURN_VALUE        

import inspect

inspect.isgeneratorfunction(gen)
True

inspect.isgeneratorfunction(func)
False

So only way to satisfy @defer.inlineCallbacks is either yield a deferred from self.signals.send_catch_log_deferred(signal=signals.engine_started) or from somewhere else.