Python.Scrapy.11-scrapy-source-code-analysis-part-1

Scrapy 源代码分析系列－1 spider, spidermanager, crawler, cmdline, command

分析的源代码版本是0.24.6, url: https://github.com/DiamondStudio/scrapy/blob/0.24.6

如github 中Scrapy 源码树所示，包含的子包有:

commands, contracts, contrib, contrib_exp, core, http, selector, settings, templates, tests, utils, xlib

包含的模块有:

_monkeypatches.py, cmdline.py, conf.py, conftest.py, crawler.py, dupefilter.py, exceptions.py,

extension.py, interface.py, item.py, link.py, linkextractor.py, log.py, logformatter.py, mail.py,

middleware.py, project.py, resolver.py, responsetypes.py, shell.py, signalmanager.py, signals.py,

spider.py, spidermanager.py, squeue.py, stats.py, statscol.py, telnet.py, webservice.py

先从重要的模块进行分析。

0. scrapy依赖的第三方库或者框架

twisted

1. 模块: spider, spidermanager, crawler, cmdline, command

1.1 spider.py spidermanager.py crawler.py

spider.py定义了spider的基类: BaseSpider. 每个spider实例只能有一个crawler属性。那么crawler具备哪些功能呢?

crawler.py定义了类Crawler，CrawlerProcess。

类Crawler依赖: SignalManager, ExtensionManager, ExecutionEngine,  以及设置项STATS_CLASS、SPIDER_MANAGER_CLASS

、LOG_FORMATTER

类CrawlerProcess: 顺序地在一个进程中运行多个Crawler。依赖: twisted.internet.reactor、twisted.internet.defer。

启动爬行(Crawlering)。该类在1.2中cmdline.py会涉及。

spidermanager.py定义类SpiderManager, 类SpiderManager用来创建和管理所有website-specific的spider。

 class SpiderManager(object):

     implements(ISpiderManager)

     def __init__(self, spider_modules):
         self.spider_modules = spider_modules
         self._spiders = {}
         for name in self.spider_modules:
             for module in walk_modules(name):
                 self._load_spiders(module)

     def _load_spiders(self, module):
         for spcls in iter_spider_classes(module):
             self._spiders[spcls.name] = spcls

1.2 cmdline.py command.py

cmdline.py定义了公有函数: execute(argv=None, settings=None)。

函数execute是工具scrapy的入口方法(entry method)，如下所示:

 XiaoKL$ cat `which scrapy`
 #!/usr/bin/python

 # -*- coding: utf-8 -*-
 import re
 import sys

 from scrapy.cmdline import execute

 if __name__ == '__main__':
     sys.argv[0] = re.sub(r'(-script\.pyw|\.exe)?$', '', sys.argv[0])
     sys.exit(execute())

所以可以根据这个点为切入点进行scrapy源码的分析。下面是execute()函数:

 def execute(argv=None, settings=None):
     if argv is None:
         argv = sys.argv

     # --- backwards compatibility for scrapy.conf.settings singleton ---
     if settings is None and 'scrapy.conf' in sys.modules:
         from scrapy import conf
         if hasattr(conf, 'settings'):
             settings = conf.settings
     # ------------------------------------------------------------------

     if settings is None:
         settings = get_project_settings()
     check_deprecated_settings(settings)

     # --- backwards compatibility for scrapy.conf.settings singleton ---
     import warnings
     from scrapy.exceptions import ScrapyDeprecationWarning
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", ScrapyDeprecationWarning)
         from scrapy import conf
         conf.settings = settings
     # ------------------------------------------------------------------

     inproject = inside_project()
     cmds = _get_commands_dict(settings, inproject)
     cmdname = _pop_command_name(argv)
     parser = optparse.OptionParser(formatter=optparse.TitledHelpFormatter(), \
         conflict_handler='resolve')
     if not cmdname:
         _print_commands(settings, inproject)
         sys.exit(0)
     elif cmdname not in cmds:
         _print_unknown_command(settings, cmdname, inproject)
         sys.exit(2)

     cmd = cmds[cmdname]
     parser.usage = "scrapy %s %s" % (cmdname, cmd.syntax())
     parser.description = cmd.long_desc()
     settings.setdict(cmd.default_settings, priority='command')
     cmd.settings = settings
     cmd.add_options(parser)
     opts, args = parser.parse_args(args=argv[1:])
     _run_print_help(parser, cmd.process_options, args, opts)

     cmd.crawler_process = CrawlerProcess(settings)
     _run_print_help(parser, _run_command, cmd, args, opts)
     sys.exit(cmd.exitcode)

execute()函数主要做: 对命令行进行解析并对scrapy命令模块进行加载；解析命令行参数；获取设置信息；创建CrawlerProcess对象。

CrawlerProcess对象、设置信息、命令行参数都赋值给ScrapyCommand(或其子类)的对象。

自然我们需要来查看定义类ScrapyCommand的模块: command.py。

ScrapyCommand的子类在子包scrapy.commands中进行定义。

_run_print_help() 函数最终调用cmd.run()，来执行该命令。如下:

 def _run_print_help(parser, func, *a, **kw):
     try:
         func(*a, **kw)
     except UsageError as e:
         if str(e):
             parser.error(str(e))
         if e.print_help:
             parser.print_help()
         sys.exit(2)

func是参数_run_command，该函数的实现主要就是调用cmd.run()方法:

 def _run_command(cmd, args, opts):
     if opts.profile or opts.lsprof:
         _run_command_profiled(cmd, args, opts)
     else:
         cmd.run(args, opts)

我们在进行设计时可以参考这个cmdline/commands无关的设计。

command.py: 定义类ScrapyCommand，该类作为Scrapy Commands的基类。来简单看一下类ScrapyCommand提供的接口/方法:

 class ScrapyCommand(object):

     requires_project = False
     crawler_process = None

     # default settings to be used for this command instead of global defaults
     default_settings = {}

     exitcode = 0

     def __init__(self):
         self.settings = None  # set in scrapy.cmdline

     def set_crawler(self, crawler):
         assert not hasattr(self, '_crawler'), "crawler already set"
         self._crawler = crawler

     @property
     def crawler(self):
         warnings.warn("Command's default `crawler` is deprecated and will be removed. "
             "Use `create_crawler` method to instatiate crawlers.",
             ScrapyDeprecationWarning)

         if not hasattr(self, '_crawler'):
             crawler = self.crawler_process.create_crawler()

             old_start = crawler.start
             self.crawler_process.started = False

             def wrapped_start():
                 if self.crawler_process.started:
                     old_start()
                 else:
                     self.crawler_process.started = True
                     self.crawler_process.start()

             crawler.start = wrapped_start

             self.set_crawler(crawler)

         return self._crawler

     def syntax(self):

     def short_desc(self):

     def long_desc(self):

     def help(self):

     def add_options(self, parser):

     def process_options(self, args, opts):

     def run(self, args, opts):

类ScrapyCommand的类属性:

requires_project: 是否需要在Scrapy project中运行

crawler_process：CrawlerProcess对象。在cmdline.py的execute()函数中进行设置。

类ScrapyCommand的方法，重点关注:

def crawler(self): 延迟创建Crawler对象。

def run(self, args, opts): 需要子类进行覆盖实现。

那么我们来具体看一个ScrapyCommand的子类(参考 Python.Scrapy.14-scrapy-source-code-analysis-part-4)。

To Be Continued:

接下来分析模块: signals.py signalmanager.py project.py conf.py Python.Scrapy.12-scrapy-source-code-analysis-part-2