Request doesn't set callback to self.parse automatically when errback is set

[joaquingx]

Hi, I'm facing an AssertionError when set errback and not callback in Request, is that expected? :

...
    def start_requests(self):
        yield Request(
            self.start_url,
            # callback=self.parse,
            errback=self.parse_error,
            meta={
                'dont_redirect': True,
            },
        )
...

trigger this :

2019-01-16 15:47:22 [scrapy.core.engine] ERROR: Error while obtaining start requests
Traceback (most recent call last):
  File "/home/joaquin/Repos/example/env/lib/python3.6/site-packages/scrapy/core/engine.py", line 127, in _next_request
    request = next(slot.start_requests)
  File "/home/joaquin/Repos/example/example/spiders/br/rj/example.py", line 128, in start_requests
    'dont_redirect': True,
  File "/home/joaquin/Repos/example/env/lib/python3.6/site-packages/scrapy/http/request/__init__.py", line 34, in __init__
    assert callback or not errback, "Cannot use errback without a callback"
AssertionError: Cannot use errback without a callback
2019-01-16 15:47:22 [scrapy.core.engine] INFO: Closing spider (finished)
2019-01-16 15:47:22 [scrapy.core.engine] INFO: Spider closed (finished)

[RomiVu]

when errback is set, callback must be set explicitly.

[joaquingx]

Hi @RomiVu, thanks for help 🙏. I cannot found a clarification about that in documentation, what I found so far:

callback (callable) – the function that will be called with the response of this request (once its downloaded) as its first parameter. For more information see Passing additional data to callback functions below. If a Request doesn’t specify a callback, the spider’s parse() method will be used. Note that if exceptions are raised during processing, errback is called instead
From https://doc.scrapy.org/en/latest/topics/request-response.html.

If it's expected that ever errback is set also callback must be set, I think that it should be documented.