虽然爬虫的工作原理我是大概清楚的，但是scrapy毕竟是个框架，要用好这个框架务必把底层结构弄清楚。
1. 去重
from __future__ import print_function
import os
import logging
from scrapy.utils.job import job_dir
from scrapy.utils.request import request_fingerprint
class BaseDupeFilter(object):
@classmethod
def from_settings(cls, settings):
return cls()
def request_seen(self, request):
return False
def open(self): # can return deferred
pass
def close(self, reason): # can return a deferred
pass
def log(self, request, spider): # log that a request has been filtered
pass
class RFPDupeFilter(BaseDupeFilter):
"""Request Fingerprint duplicates filter"""
def __init__(self, path=None, debug=False):
self.file = None
self.fingerprints = set()#用集合来记录已经访问的request
self.logdupes = True
self.debug = debug
self.logger = logging.getLogger(__name__)
if path:
self.file = open(os.path.join(path, 'requests.seen'), 'a+')
self.file.seek(0)
self.fingerprints.update(x.rstrip() for x in self.file)
@classmethod
def from_settings(cls, settings):
debug = settings.getbool('DUPEFILTER_DEBUG')
return cls(job_dir(settings), debug)
def request_seen(self, request):
fp = self.request_fingerprint(request)
if fp in self.fingerprints:
return True
self.fingerprints.add(fp)
if self.file:
self.file.write(fp + os.linesep)
def request_fingerprint(self, request):
return request_fingerprint(request)
def close(self, reason):
if self.file:
self.file.close()
def log(self, request, spider):
if self.debug:
msg = "Filtered duplicate request: %(request)s"
self.logger.debug(msg, {'request': request}, extra={'spider': spider})
elif self.logdupes:
msg = ("Filtered duplicate request: %(request)s"
" - no more duplicates will be shown"
" (see DUPEFILTER_DEBUG to show all duplicates)")
self.logger.debug(msg, {'request': request}, extra={'spider': spider})
self.logdupes = False
spider.crawler.stats.inc_value('dupefilter/filtered', spider=spider)
上面的代码是系统自带的去重，代码中使用set来去重，这跟我以前单独写的代码是一样的。那么系统在哪调用去重代码呢？
#core/scheduler.py
def enqueue_request(self, request):
if not request.dont_filter and self.df.request_seen(request):
self.df.log(request, self.spider)
return False
dqok = self._dqpush(request)
if dqok:
self.stats.inc_value('scheduler/enqueued/disk', spider=self.spider)
else:
self._mqpush(request)
self.stats.inc_value('scheduler/enqueued/memory', spider=self.spider)
self.stats.inc_value('scheduler/enqueued', spider=self.spider)
return True
看上面的代码self.df.request_seen，这句调用就是去重类中的函数。我们在返回去看这个函数，发现这个函数与我们处理方式是不同的，request_fingerprint这个函数用来计算请求指纹，对于同一个链接，如果使用cookie来传递参数或者使用post来传递参数，那么我以前的去重方法是不对的。
这个去重类中还有个功能就是能中断之后继续工作，这需要在settings设置JOBDIR。
可以在settings配置自定义的类DUPEFILTER_CLASS。
