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