Python.Scrapy.14-scrapy-source-code-analysis-part-4

Scrapy 源代码分析系列－4 scrapy.commands 子包

子包scrapy.commands定义了在命令scrapy中使用的子命令(subcommand): bench, check, crawl, deploy, edit, fetch,

genspider, list, parse, runspider, settings, shell, startproject, version, view。 所有的子命令模块都定义了一个继承自

类ScrapyCommand的子类Command。

首先来看一下子命令crawl, 该子命令用来启动spider。

1. crawl.py

关注的重点在方法run(self, args, opts):

 def run(self, args, opts):
         if len(args) < 1:
             raise UsageError()
         elif len(args) > 1:
             raise UsageError("running 'scrapy crawl' with more than one spider is no longer supported")
         spname = args[0]

         crawler = self.crawler_process.create_crawler()  # A
         spider = crawler.spiders.create(spname, **opts.spargs) # B
         crawler.crawl(spider) ＃ C
         self.crawler_process.start() # D

那么问题来啦，run接口方法是从哪里调用的呢? 让我们回到 Python.Scrapy.11-scrapy-source-code-analysis-part-1

中 "1.2 cmdline.py command.py" 关于"_run_print_help() "的说明。

A: 创建类Crawler对象crawler。在创建Crawler对象时, 同时将创建Crawler对象的实例属性spiders(SpiderManager)。如下所示:

 class Crawler(object):

     def __init__(self, settings):
         self.configured = False
         self.settings = settings
         self.signals = SignalManager(self)
         self.stats = load_object(settings['STATS_CLASS'])(self)
         self._start_requests = lambda: ()
         self._spider = None
         # TODO: move SpiderManager to CrawlerProcess
         spman_cls = load_object(self.settings['SPIDER_MANAGER_CLASS'])
         self.spiders = spman_cls.from_crawler(self)  # spiders 的类型是: SpiderManager

Crawler对象对应一个SpiderManager对象，而SpiderManager对象管理多个Spider。

B: 获取Sipder对象。

C: 为Spider对象安装Crawler对象。(为蜘蛛安装爬行器)

D: 类CrawlerProcess的start()方法如下:

     def start(self):
         if self.start_crawling():
             self.start_reactor()

     def start_crawling(self):
         log.scrapy_info(self.settings)
         return self._start_crawler() is not None

     def start_reactor(self):
         if self.settings.getbool('DNSCACHE_ENABLED'):
             reactor.installResolver(CachingThreadedResolver(reactor))
         reactor.addSystemEventTrigger('before', 'shutdown', self.stop)
         reactor.run(installSignalHandlers=False)  # blocking call

     def _start_crawler(self):
         if not self.crawlers or self.stopping:
             return

         name, crawler = self.crawlers.popitem()
         self._active_crawler = crawler
         sflo = log.start_from_crawler(crawler)
         crawler.configure()
         crawler.install()
         crawler.signals.connect(crawler.uninstall, signals.engine_stopped)
         if sflo:
             crawler.signals.connect(sflo.stop, signals.engine_stopped)
         crawler.signals.connect(self._check_done, signals.engine_stopped)
         crawler.start()  ＃ 调用类Crawler的start()方法
         return name, crawler

类Crawler的start()方法如下:

     def start(self):
         yield defer.maybeDeferred(self.configure)
         if self._spider:
             yield self.engine.open_spider(self._spider, self._start_requests()) # 和Engine建立了联系 （ExecutionEngine）
         yield defer.maybeDeferred(self.engine.start)

关于类ExecutionEngine将在子包scrapy.core分析涉及。

2. startproject.py

3. subcommand是如何加载的

在cmdline.py的方法execute()中有如下几行代码:

     inproject = inside_project()
     cmds = _get_commands_dict(settings, inproject)
     cmdname = _pop_command_name(argv)

_get_commands_dict():

 def _get_commands_dict(settings, inproject):
     cmds = _get_commands_from_module('scrapy.commands', inproject)
     cmds.update(_get_commands_from_entry_points(inproject))
     cmds_module = settings['COMMANDS_MODULE']
     if cmds_module:
         cmds.update(_get_commands_from_module(cmds_module, inproject))
     return cmds

_get_commands_from_module():

 def _get_commands_from_module(module, inproject):
     d = {}
     for cmd in _iter_command_classes(module):
         if inproject or not cmd.requires_project:
             cmdname = cmd.__module__.split('.')[-1]
             d[cmdname] = cmd()
     return d

To Be Continued

接下来解析settings相关的逻辑。Python.Scrapy.15-scrapy-source-code-analysis-part-5