How do I handle errors and exceptions in Scrapy?

Scraping websites can get messy due to the unpredictable nature of the internet. Websites might go down, requests might timeout, or you might even get blocked. To deal with such issues, Scrapy provides a built-in mechanism for handling errors and exceptions.

Here is how you can handle errors and exceptions in Scrapy:

Handling HTTP errors

Scrapy uses the HttpError middleware to handle HTTP error codes. By default, Scrapy ignores the HTTP codes mentioned in the handle_httpstatus_list spider attribute and the HTTPERROR_ALLOWED_CODES setting.

If you want to handle certain HTTP codes, you can set the handle_httpstatus_list attribute in your spider:

class MySpider(scrapy.Spider):
    name = 'myspider'
    handle_httpstatus_list = [404]

    # Your spider definition here...

In this case, Scrapy won't ignore 404 errors and will pass the response to the spider.

Handling exceptions in spider callbacks

You can handle exceptions in your spider callbacks by using a try/except block:

class MySpider(scrapy.Spider):
    name = 'myspider'

    def parse(self, response):
        try:
            # Your parsing code here...
        except Exception as e:
            self.logger.error(f"An error occurred: {e}")

The above code will catch any exception occurred in the parsing code and will log it.

Handling download errors

Scrapy provides a method handle_exception in the Request object to handle download errors. You can override this method in your spider to handle download errors:

def handle_exception(self, exception):
    self.logger.error(f"A download error occurred: {exception}")

Handling spider errors

Scrapy sends the spider_error signal when an error occurs in the spider. You can catch this signal and handle the error:

from scrapy import signals

class MySpider(scrapy.Spider):
    name = 'myspider'

    @classmethod
    def from_crawler(cls, crawler, *args, **kwargs):
        spider = super().from_crawler(crawler, *args, **kwargs)
        crawler.signals.connect(spider.handle_spider_error, signal=signals.spider_error)
        return spider

    def handle_spider_error(self, failure, response, spider):
        self.logger.error(f"A spider error occurred: {failure}")

In the above code, the handle_spider_error method will be called when a spider error occurs.

Remember that handling errors and exceptions is crucial in web scraping to make your spider more resilient and capable of running for long periods.