Scrapy 框架 中间件，信号，定制命令

Posted 2021-02-08 shijieli

中间件

下载器中间件

写中间件

from scrapy.http import htmlResponse
from scrapy.http import Request

class Md1(object):
    @classmethod
    def from_crawler(cls, crawler):
        # This method is used by Scrapy to create your spiders.
        s = cls()
        return s

    def process_request(self, request, spider):    
        print(‘md1.process_request‘,request)
        return None # 返回如果是 空就会继续往下执行下一个中间件的 process_request 方法，如果一旦有返回值就要考虑情况
        """
        # 1. 返回 Response
        # 返回 Response 之后会往下执行 最后一个中间件的 process_response 方法 
        # import requests
        # result = requests.get(request.url)
        # return HtmlResponse(url=request.url, status=200, headers=None, body=result.content)
        
        # 2. 返回 Request
        # 返回 Request 之后 相当于无视了这次的请求 重新回到 调制器 那边，相当于又产生了新的任务
        # return Request(‘https://dig.chouti.com/r/tec/hot/1‘)

        # 3. 抛出异常    
        # 抛出异常 必须要 有 process_exception 方法进行捕捉异常，不然会报错
        # process_exception 方法在进行一系列的操作 在捕捉到异常的时候 
        # from scrapy.exceptions import IgnoreRequest
        # raise IgnoreRequest
        
        # 4. 对请求进行加工(*) 
        # 通常我们都是用于对请求加工，然后再继续下面操作不返回东西 
        # request.headers[‘user-agent‘] = "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36"
        # return None
        """

    def process_response(self, request, response, spider):
        # Called with the response returned from the downloader.

        # Must either;
        # - return a Response object    # 返回一个 Response 来代替当前的 Response
        # - return a Request object        # 返回一个 Request 开启新任务 
        # - or raise IgnoreRequest        # 返回一个 IgnoreRequest 进行异常捕捉 
        print(‘m1.process_response‘,request,response)
        return response

    def process_exception(self, request, exception, spider):
        # Called when a download handler or a process_request()
        # (from other downloader middleware) raises an exception.

        # Must either:
        # - return None: continue processing this exception
            # 通常我们都是直接返回 None 就可以了
        # - return a Response object: stops process_exception() chain    # 只要返回了 Response 当前的 process_exception 就不做操作了 
            # 返回 Response 表示交给下一个 中间件的 process_exception 继续处理 
        # - return a Request object: stops process_exception() chain    # 只要返回了 Request 当前的 process_exception 就不做操作了 
            # 返回 Request 放弃本次任务，新建任务     
        pass

配置文件

DOWNLOADER_MIDDLEWARES = {    
    #‘xdb.middlewares.XdbDownloaderMiddleware‘: 543,
    # ‘xdb.proxy.XdbProxyMiddleware‘:751,
    ‘xdb.md.Md1‘:666,    # 依旧是 0-1000 越小越优先 
    ‘xdb.md.Md2‘:667,
}

执行顺序梳理

调度器 给 下载器的时候先走 process_request（从第一个中间件往最后一个走） 然后如果根据返回情况进行判断接下来的方向
　　返回 None 继续下一个中间件的 process_request
　　返回 Response 进入 最后一个下载中间件的 process_response 流程
　　返回 Request 返回 调度器开启新任务 
　　返回 异常  进入当前中间件的 process_exception 进行异常处理

下载器 还给 爬虫的时候要走 process_response（从最后一个中间件往第一个走）然后如果根据返回情况进行判断接下来的方向
　　返回 None 继续上一个中间件的 process_response
　　返回 Response 替换当前Response 进入上一个下载中间件的 process_response 流程
　　返回 Request 返回 调度器开启新任务 放弃当前的任务  
　　返回 异常  进入当前中间件的 process_exception 进行异常处理

应用场景

- user-agent # 所有的请求都加 user-agent    
    # 其实不需要做，默认自带一个 可以添加 user-agent 的功能
    # 再 settings 中 USER_AGENT = ‘‘ 直接配置就可以实现这个功能 
- 代理     # 请求代理操作 

爬虫中间件

写中间件

class Sd1(object):
    # Not all methods need to be defined. If a method is not defined,
    # scrapy acts as if the spider middleware does not modify the
    # passed objects.

    @classmethod
    def from_crawler(cls, crawler):
        # This method is used by Scrapy to create your spiders.
        s = cls()
        return s

    def process_spider_input(self, response, spider):
        # Called for each response that goes through the spider
        # middleware and into the spider.

        # Should return None or raise an exception.
        return None

    def process_spider_output(self, response, result, spider):
        # Called with the results returned from the Spider, after
        # it has processed the response.

        # Must return an iterable of Request, dict or Item objects.
        for i in result:
            yield i

    def process_spider_exception(self, response, exception, spider):
        # Called when a spider or process_spider_input() method
        # (from other spider middleware) raises an exception.

        # Should return either None or an iterable of Response, dict
        # or Item objects.
        pass

    # 只在爬虫启动时，执行一次。
    def process_start_requests(self, start_requests, spider):
        # Called with the start requests of the spider, and works
        # similarly to the process_spider_output() method, except
        # that it doesn’t have a response associated.

        # Must return only requests (not items).
        for r in start_requests:
            yield r

配置文件

SPIDER_MIDDLEWARES = {
   # ‘xdb.middlewares.XdbSpiderMiddleware‘: 543,
    ‘xdb.sd.Sd1‘: 666,    # 同爬虫中间件一样的判断机制 
    ‘xdb.sd.Sd2‘: 667,
}

执行流程

1. 第一次启动爬虫文件封装好 request 之后 走 process_start_requests 上传给引擎

2. 引擎将封装好的 request 给调度器

3. 调度器 继续执行 给下载器

4. 下载器 下载了内容之后给 引擎

5. 引擎再给 爬虫文件的时候要走 process_spider_input

6. 爬虫文件处理完之后如果有 yield 就要在走 process_spider_output 给引擎

应用

- 深度

- 优先级

信号

使用框架预留的位置，帮助你自定义一些功能

使用实例

from scrapy import signals

class MyExtend(object):
    def __init__(self):
        pass

    @classmethod
    def from_crawler(cls, crawler):
        self = cls()

        crawler.signals.connect(self.x1, signal=signals.spider_opened) # 绑定信号发生时允许的函数
        crawler.signals.connect(self.x2, signal=signals.spider_closed)

        return self

    def x1(self, spider):
        print(‘open‘)

    def x2(self, spider):
        print(‘close‘)

 

# 信号可选类型  from scrapy import signals 中可以看到 
engine_started = object()
engine_stopped = object()

spider_opened = object()
spider_idle = object()
spider_closed = object()
spider_error = object()

request_scheduled = object()
request_dropped = object()
response_received = object()
response_downloaded = object()

item_scraped = object()
item_dropped = object()

 

# settings.py 

EXTENSIONS = {
            ‘xdb.ext.MyExtend‘:666,
        }

定制命令

单爬虫运行

import sys
from scrapy.cmdline import execute

if __name__ == ‘__main__‘:
    execute(["scrapy","crawl","chouti","--nolog"])

所有爬虫

- 在spiders同级创建任意目录，如：commands
- 在其中创建 crawlall.py 文件 （此处文件名就是自定义的命令）
- 在settings.py 中添加配置 COMMANDS_MODULE = ‘项目名称.目录名称‘
- 在项目目录执行命令：scrapy crawlall 

# crawlall.py

from scrapy.commands import ScrapyCommand
from scrapy.utils.project import get_project_settings


class Command(ScrapyCommand):

    requires_project = True

    def syntax(self):
        return ‘[options]‘

    def short_desc(self):
        return ‘Runs all of the spiders‘

    def run(self, args, opts):
        spider_list = self.crawler_process.spiders.list()
        for name in spider_list:
            self.crawler_process.crawl(name, **opts.__dict__)
        self.crawler_process.start()

# settings.py

COMMANDS_MODULE  = "xdb.commands"