Coming from C++ background, trying to understand what callback is doing in this function from the Scrapy...





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I'm following the basic Scrapy tutorial, and have some limited python experience. This seems like a recursive function, and I have some questions about what is happening.



This is in the Scrapy tutorial: https://doc.scrapy.org/en/latest/intro/tutorial.html



This ran the same when I specified callback=self.parse and when I left it out.



Here's the code (the last line is where my question is coming from):



def parse(self, response):

    for quote in response.css('div.quote'):

        yield {

            'text':     quote.css('span.text::text').extract_first(),

            'author':     quote.css('small.author::text').extract_first(),

            'tags': quote.css('div.tags a.tag::text').extract(),

        }



    next_page = response.css('li.next a::attr(href)').extract_first()

    if next_page is not None:

        next_page = response.urljoin(next_page)

        yield scrapy.Request(next_page, callback=self.parse)


The function performs identically when I omit callback=self.parse and when I leave it in.
Is this callback implicit, and not necessary? Is there a reason you need to have it in there?



Thanks in advance.






python web-scraping scrapy







share|improve this question




















  • 1





    I read this somewhere in the scrapy documentation If callback is None follow defaults to True , otherwise it defaults to False. Even if you do not explicitly define callback within your script, It will still follow the default function which is parse() in this case. Hope it clears your confusion.

    – robots.txt
    Jan 3 at 9:38


















0















I'm following the basic Scrapy tutorial, and have some limited python experience. This seems like a recursive function, and I have some questions about what is happening.



This is in the Scrapy tutorial: https://doc.scrapy.org/en/latest/intro/tutorial.html



This ran the same when I specified callback=self.parse and when I left it out.



Here's the code (the last line is where my question is coming from):



def parse(self, response):

    for quote in response.css('div.quote'):

        yield {

            'text':     quote.css('span.text::text').extract_first(),

            'author':     quote.css('small.author::text').extract_first(),

            'tags': quote.css('div.tags a.tag::text').extract(),

        }



    next_page = response.css('li.next a::attr(href)').extract_first()

    if next_page is not None:

        next_page = response.urljoin(next_page)

        yield scrapy.Request(next_page, callback=self.parse)


The function performs identically when I omit callback=self.parse and when I leave it in.
Is this callback implicit, and not necessary? Is there a reason you need to have it in there?



Thanks in advance.






python web-scraping scrapy







share|improve this question




















  • 1





    I read this somewhere in the scrapy documentation If callback is None follow defaults to True , otherwise it defaults to False. Even if you do not explicitly define callback within your script, It will still follow the default function which is parse() in this case. Hope it clears your confusion.

    – robots.txt
    Jan 3 at 9:38














0












0








0








I'm following the basic Scrapy tutorial, and have some limited python experience. This seems like a recursive function, and I have some questions about what is happening.



This is in the Scrapy tutorial: https://doc.scrapy.org/en/latest/intro/tutorial.html



This ran the same when I specified callback=self.parse and when I left it out.



Here's the code (the last line is where my question is coming from):



def parse(self, response):

    for quote in response.css('div.quote'):

        yield {

            'text':     quote.css('span.text::text').extract_first(),

            'author':     quote.css('small.author::text').extract_first(),

            'tags': quote.css('div.tags a.tag::text').extract(),

        }



    next_page = response.css('li.next a::attr(href)').extract_first()

    if next_page is not None:

        next_page = response.urljoin(next_page)

        yield scrapy.Request(next_page, callback=self.parse)


The function performs identically when I omit callback=self.parse and when I leave it in.
Is this callback implicit, and not necessary? Is there a reason you need to have it in there?



Thanks in advance.






python web-scraping scrapy







share|improve this question
















I'm following the basic Scrapy tutorial, and have some limited python experience. This seems like a recursive function, and I have some questions about what is happening.



This is in the Scrapy tutorial: https://doc.scrapy.org/en/latest/intro/tutorial.html



This ran the same when I specified callback=self.parse and when I left it out.



Here's the code (the last line is where my question is coming from):



def parse(self, response):

    for quote in response.css('div.quote'):

        yield {

            'text':     quote.css('span.text::text').extract_first(),

            'author':     quote.css('small.author::text').extract_first(),

            'tags': quote.css('div.tags a.tag::text').extract(),

        }



    next_page = response.css('li.next a::attr(href)').extract_first()

    if next_page is not None:

        next_page = response.urljoin(next_page)

        yield scrapy.Request(next_page, callback=self.parse)


The function performs identically when I omit callback=self.parse and when I leave it in.
Is this callback implicit, and not necessary? Is there a reason you need to have it in there?



Thanks in advance.






python web-scraping scrapy




python web-scraping scrapy






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Jan 3 at 11:02









stranac

14.8k31725




14.8k31725










asked Jan 3 at 9:20









TrevorTrevor

7




7








  • 1





    I read this somewhere in the scrapy documentation If callback is None follow defaults to True , otherwise it defaults to False. Even if you do not explicitly define callback within your script, It will still follow the default function which is parse() in this case. Hope it clears your confusion.

    – robots.txt
    Jan 3 at 9:38














  • 1





    I read this somewhere in the scrapy documentation If callback is None follow defaults to True , otherwise it defaults to False. Even if you do not explicitly define callback within your script, It will still follow the default function which is parse() in this case. Hope it clears your confusion.

    – robots.txt
    Jan 3 at 9:38








1




1





I read this somewhere in the scrapy documentation If callback is None follow defaults to True , otherwise it defaults to False. Even if you do not explicitly define callback within your script, It will still follow the default function which is parse() in this case. Hope it clears your confusion.

– robots.txt
Jan 3 at 9:38





I read this somewhere in the scrapy documentation If callback is None follow defaults to True , otherwise it defaults to False. Even if you do not explicitly define callback within your script, It will still follow the default function which is parse() in this case. Hope it clears your confusion.

– robots.txt
Jan 3 at 9:38












1 Answer
1






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oldest

votes


















1














The documentation you linked explains what's happening in the A shortcut to the start_requests method section:




parse() is Scrapy’s default callback method, which is called for requests without an explicitly assigned callback




The scrapy tutorial just shows the basic method, and then tries to ease you into using alternatives.






share|improve this answer
























  • The reason the documentation leaves it in is so that you understand that you can change the callback name there to use a different callback. Otherwise, someone reading the documentation might wonder how to use a different callback, or worse, think it is not possible to use a different callback.

    – Gallaecio
    Jan 16 at 11:33












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1 Answer
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active

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









1














The documentation you linked explains what's happening in the A shortcut to the start_requests method section:




parse() is Scrapy’s default callback method, which is called for requests without an explicitly assigned callback




The scrapy tutorial just shows the basic method, and then tries to ease you into using alternatives.






share|improve this answer
























  • The reason the documentation leaves it in is so that you understand that you can change the callback name there to use a different callback. Otherwise, someone reading the documentation might wonder how to use a different callback, or worse, think it is not possible to use a different callback.

    – Gallaecio
    Jan 16 at 11:33
















1














The documentation you linked explains what's happening in the A shortcut to the start_requests method section:




parse() is Scrapy’s default callback method, which is called for requests without an explicitly assigned callback




The scrapy tutorial just shows the basic method, and then tries to ease you into using alternatives.






share|improve this answer
























  • The reason the documentation leaves it in is so that you understand that you can change the callback name there to use a different callback. Otherwise, someone reading the documentation might wonder how to use a different callback, or worse, think it is not possible to use a different callback.

    – Gallaecio
    Jan 16 at 11:33














1












1








1







The documentation you linked explains what's happening in the A shortcut to the start_requests method section:




parse() is Scrapy’s default callback method, which is called for requests without an explicitly assigned callback




The scrapy tutorial just shows the basic method, and then tries to ease you into using alternatives.






share|improve this answer













The documentation you linked explains what's happening in the A shortcut to the start_requests method section:




parse() is Scrapy’s default callback method, which is called for requests without an explicitly assigned callback




The scrapy tutorial just shows the basic method, and then tries to ease you into using alternatives.







share|improve this answer












share|improve this answer



share|improve this answer










answered Jan 3 at 10:59









stranacstranac

14.8k31725




14.8k31725













  • The reason the documentation leaves it in is so that you understand that you can change the callback name there to use a different callback. Otherwise, someone reading the documentation might wonder how to use a different callback, or worse, think it is not possible to use a different callback.

    – Gallaecio
    Jan 16 at 11:33



















  • The reason the documentation leaves it in is so that you understand that you can change the callback name there to use a different callback. Otherwise, someone reading the documentation might wonder how to use a different callback, or worse, think it is not possible to use a different callback.

    – Gallaecio
    Jan 16 at 11:33

















The reason the documentation leaves it in is so that you understand that you can change the callback name there to use a different callback. Otherwise, someone reading the documentation might wonder how to use a different callback, or worse, think it is not possible to use a different callback.

– Gallaecio
Jan 16 at 11:33





The reason the documentation leaves it in is so that you understand that you can change the callback name there to use a different callback. Otherwise, someone reading the documentation might wonder how to use a different callback, or worse, think it is not possible to use a different callback.

– Gallaecio
Jan 16 at 11:33




















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