How do I debug a Scrapy spider?

Debugging a Scrapy spider can be done in several ways. The method of debugging often depends on the kind of issue you're experiencing. Here are some general steps and techniques that you can use to debug your Scrapy spiders:

1. Logging: Scrapy has inbuilt support for logging which can be highly useful in tracking the progress and debugging. You can set the logging level to 'DEBUG' to see all the details about your crawl.

   import logging
   
   class MySpider(scrapy.Spider):
       name = 'myspider'
       start_urls = ['http://example.com']
   
       def parse(self, response):
           self.log('Visited %s' % response.url)
   
       def close(self, reason):
           self.log('spider closed: %s' % reason, level=logging.INFO)
   

   You can also set the log level in your settings.py file or when you run the spider with the -s or --set option:

   scrapy crawl myspider -s LOG_LEVEL=INFO
   

2. Python Debugger (PDB): You can use Python's built-in debugger pdb. You can put import ipdb; ipdb.set_trace() at any place in your spider to start an ipdb session at that point.

   import scrapy
   import ipdb;
   
   class MySpider(scrapy.Spider):
       name = 'myspider'
       start_urls = ['http://example.com']
   
       def parse(self, response):
           ipdb.set_trace()
           self.log('Visited %s' % response.url)
   

   When the spider hits the ipdb line, it will stop and give you a debug prompt. You can then inspect variables, statuses, etc.

3. Scrapy Shell: The Scrapy shell is an interactive shell where you can try and debug your scraping code very quickly, without having to run the spider. You can use the Scrapy shell like this:

   scrapy shell 'http://example.com'
   

   This will fetch the page from the URL and provide you with response containing the page's content. You can now run your XPath or CSS selectors to debug your parsing code.

4. Unit Testing: Another way to debug your spiders is to write unit tests for them. Scrapy uses the built-in unittest module of Python.

   import unittest
   from myproject.spiders import MySpider
   
   class TestMySpider(unittest.TestCase):
       def setUp(self):
           self.spider = MySpider()
   
       def test_parse(self):
           # Your test code here
   

   This way, you can make sure that individual units of your spider are working as expected.

Remember, the best way to debug depends on the issue, and often it's best to combine these methods. It's always good to have detailed logs, and sometimes it's faster to test your code in the Scrapy shell before running the full spider. Debugging with pdb is especially useful for complex issues that can't be solved by the other methods.