scrapy ,Why does the scrapy.Request class call the parse() method by default?

Question

Why does the scrapy.Request class call the parse() method by default, and I do not quite understand the process

the part of scrapy.Request source code

class Request(object_ref):

def __init__(self, url, callback=None, method='GET', headers=None, body=None,
             cookies=None, meta=None, encoding='utf-8', priority=0,
             dont_filter=False, errback=None, flags=None):

    self._encoding = encoding  # this one has to be set first
    self.method = str(method).upper()
    self._set_url(url)
    self._set_body(body)
    assert isinstance(priority, int), "Request priority not an integer: %r" % priority
    self.priority = priority

    assert callback or not errback, "Cannot use errback without a callback"
    self.callback = callback
    self.errback = errback

....

but this default callback is None so I am very puzzled by this

    if "msg" in text_json and text_json["msg"] == "login":
        for url in self.start_urls:
            yield scrapy.Request(url, dont_filter=True, headers=self.headers)

Answer 1

This is something that is decided inside the Scrapy core , see this request.callback or spider.parse part:

def call_spider(self, result, request, spider):
    result.request = request
    dfd = defer_result(result)
    dfd.addCallbacks(request.callback or spider.parse, request.errback)
    return dfd.addCallback(iterate_spider_output)