scrapy 浅分析(一)

0x00.前言

相信大家都用过python的scrapy框架写给爬虫。然后就想看看这个框架的简单执行流程。

0x01.大体架构

官方架构图如下

图大体架构很清晰，实际以例子来看看，一般我们的爬虫以如下代码开始运行(命令行是一样的)

from scrapy import cmdline

cmdline.execute("scrapy crawl xxx".split())

找到scrapy安装的包文件，上述代码即为运行cmdline.py文件里面的execute()函数，该函数较长，阶段看

如下：

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
    # ------------------------------------------------------------------

这里得到外部参数，然后即为注释所说为兼容性操作,此处继续为cmdline.py文件。

if settings is None:
     settings = get_project_settings()
     # set EDITOR from environment if available
     try:
         editor = os.environ['EDITOR']
     except KeyError: pass
     else:
         settings['EDITOR'] = editor
 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
 # ------------------------------------------------------------------

执行get_project_settings()函数，全局寻找跟进得，此处为utils文件夹下的project.py文件：

def get_project_settings():
  #ENVVAR = 'SCRAPY_SETTINGS_MODULE',变量所在文件中已经定义了
    if ENVVAR not in os.environ:
        project = os.environ.get('SCRAPY_PROJECT', 'default')
        #project此时为default，下面进行初始化
        init_env(project)

Init_env()函数如下：

def init_env(project='default', set_syspath=True):
    """Initialize environment to use command-line tool from inside a project
    dir. This sets the Scrapy settings module and modifies the Python path to
    be able to locate the project module.
    """
    cfg = get_config()
    if cfg.has_option('settings', project):
        os.environ['SCRAPY_SETTINGS_MODULE'] = cfg.get('settings', project)
    closest = closest_scrapy_cfg()
    if closest:
        projdir = os.path.dirname(closest)
        if set_syspath and projdir not in sys.path:
            sys.path.append(projdir)

如注释所说，初始化环境,循环递归找到用户项目中的配置文件settings.py,并且将其设置到环境变量SCRAPY_SETTINGS_MODULE中。然后继续get_project_settings函数：

#创建settings实例，根据包导入情况，from scrapy.settings import Settings，进入settings文件夹可发 #现此过程同时加载默认配置文件default_settings.pys
 	settings = Settings()
   #得到用户配置文件路径
   settings_module_path = os.environ.get(ENVVAR)
   #更新配置，有则覆盖
   if settings_module_path:
       settings.setmodule(settings_module_path, priority='project')
#以下都为一些更新操作，setdict函数里面是一些set_update函数
   # XXX: remove this hack
   pickled_settings = os.environ.get("SCRAPY_PICKLED_SETTINGS_TO_OVERRIDE")
   if pickled_settings:
       settings.setdict(pickle.loads(pickled_settings), priority='project')

   # XXX: deprecate and remove this functionality
   env_overrides = {k[7:]: v for k, v in os.environ.items() if
                    k.startswith('SCRAPY_')}
   if env_overrides:
       settings.setdict(env_overrides, priority='project')

   return settings

至此，get_project_settings()该函数结束，如函数名字一样，最后返回项目配置。回到最初cmdline.py文件

	#容差兼容性检测
 	check_deprecated_settings(settings)
#兼容
   # --- 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
   # ------------------------------------------------------------------

此处执行check_deprecated_settings函数

def check_deprecated_settings(settings):
    deprecated = [x for x in DEPRECATED_SETTINGS if settings[x[0]] is not None]
    if deprecated:
        msg = "You are using the following settings which are deprecated or obsolete"
        msg += " (ask scrapy-users@googlegroups.com for alternatives):"
        msg = msg + "\n    " + "\n    ".join("%s: %s" % x for x in deprecated)
        warnings.warn(msg, ScrapyDeprecationWarning)

做一些容差性检测。接下来cmdline.py函数execute()代码如下

#是否环境在项目中	
	inproject = inside_project()
   #做字典
   cmds = _get_commands_dict(settings, inproject)

inside_project函数如下：

def inside_project():
    scrapy_module = os.environ.get('SCRAPY_SETTINGS_MODULE')
    if scrapy_module is not None:
        try:
            import_module(scrapy_module)
        except ImportError as exc:
            warnings.warn("Cannot import scrapy settings module %s: %s" % (scrapy_module, exc))
        else:
            return True
    return bool(closest_scrapy_cfg())

检测是否有SCRAPY_SETTINGS_MODULE，这是前面有的啊，如何当真没有则执行closest_scrapy_cfg(),该函数如下：

def closest_scrapy_cfg(path='.', prevpath=None):
    """Return the path to the closest scrapy.cfg file by traversing the current
    directory and its parents
    """
    if path == prevpath:
        return ''
    path = os.path.abspath(path)
    cfgfile = os.path.join(path, 'scrapy.cfg')
    if os.path.exists(cfgfile):
        return cfgfile
    return closest_scrapy_cfg(os.path.dirname(path), path)

循环查找scrapy.cfg,如何有就返回，同时上面代码bool返回真。如函数名字所示，该inside_project函数检测环境是否于项目中。随后执行cmds = _get_commands_dict(settings, inproject),涉及如下：

def _iter_command_classes(module_name):
    # TODO: add `name` attribute to commands and and merge this function with
    # scrapy.utils.spider.iter_spider_classes
    for module in walk_modules(module_name):
        for obj in vars(module).values():
            if inspect.isclass(obj) and \
                    issubclass(obj, ScrapyCommand) and \
                    obj.__module__ == module.__name__ and \
                    not obj == ScrapyCommand:
                yield obj

def _get_commands_from_module(module, inproject):
    d = {}
    #将module也即commands文件夹文件带入
    for cmd in _iter_command_classes(module):
        if inproject or not cmd.requires_project:
            cmdname = cmd.__module__.split('.')[-1]
            d[cmdname] = cmd()
    return d

def _get_commands_from_entry_points(inproject, group='scrapy.commands'):
    cmds = {}
    for entry_point in pkg_resources.iter_entry_points(group):
        obj = entry_point.load()
        if inspect.isclass(obj):
            cmds[entry_point.name] = obj()
        else:
            raise Exception("Invalid entry point %s" % entry_point.name)
    return cmds

def _get_commands_dict(settings, inproject):
   #将commands文件夹带入该函数中
    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
  	#最后返回一个字典

将该函数返回结果进行打印，如下：

{'bench': <scrapy.commands.bench.Command object at 0x180d597f98>, 'check': <scrapy.commands.check.Command object at 0x180d5975c0>, 'crawl': <scrapy.commands.crawl.Command object at 0x180d57ef98>, 'edit': <scrapy.commands.edit.Command object at 0x180d57ee48>, 'fetch': <scrapy.commands.fetch.Command object at 0x180d57eb70>, 'genspider': <scrapy.commands.genspider.Command object at 0x180d57eb38>, 'list': <scrapy.commands.list.Command object at 0x180d57edd8>, 'parse': <scrapy.commands.parse.Command object at 0x180d5b1080>, 'runspider': <scrapy.commands.runspider.Command object at 0x180d5832e8>, 'settings': <scrapy.commands.settings.Command object at 0x180d5833c8>, 'shell': <scrapy.commands.shell.Command object at 0x180d5b54a8>, 'startproject': <scrapy.commands.startproject.Command object at 0x180d5b5438>, 'version': <scrapy.commands.version.Command object at 0x180d5b5470>, 'view': <scrapy.commands.view.Command object at 0x180d5b55c0>}

这些均为commands文件夹下文件，可以发现这些个文件几乎都存在run函数，都是可以执行的命令，有没有发现一个特别的crawl呢。继续

	#得到你输入的命令
 	cmdname = _pop_command_name(argv)
   parser = optparse.OptionParser(formatter=optparse.TitledHelpFormatter(), \
       conflict_handler='resolve')
   if not cmdname:
       _print_commands(settings, inproject)
       sys.exit(0)
     #字典中寻找，找到对应的命令
   elif cmdname not in cmds:
       _print_unknown_command(settings, cmdname, inproject)
       sys.exit(2)
#找到了，还有一些处理
   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)
#创建CrawlerProcess实例
   cmd.crawler_process = CrawlerProcess(settings)
   #将对应的命令执行，这里是crawl
   _run_print_help(parser, _run_command, cmd, args, opts)
   sys.exit(cmd.exitcode)

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

0x02.小结

这里一路下来就是，得到配置文件，照着输入的命令于commands文件夹中寻找对应命令并执行。这里还没有得到太多细节，下次再聊。