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。

 1 class SpiderManager(object):
 2 
 3     implements(ISpiderManager)
 4 
 5     def __init__(self, spider_modules):
 6         self.spider_modules = spider_modules
 7         self._spiders = {}
 8         for name in self.spider_modules:
 9             for module in walk_modules(name):
10                 self._load_spiders(module)
11 
12     def _load_spiders(self, module):
13         for spcls in iter_spider_classes(module):
14             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:

 1 XiaoKL$ cat `which scrapy`
 2 #!/usr/bin/python
 3 
 4 # -*- coding: utf-8 -*-
 5 import re
 6 import sys
 7 
 8 from scrapy.cmdline import execute
 9 
10 if __name__ == '__main__':
11     sys.argv[0] = re.sub(r'(-script.pyw|.exe)?$', '', sys.argv[0])
12     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:

 1 def execute(argv=None, settings=None):
 2     if argv is None:
 3         argv = sys.argv
 4 
 5     # --- backwards compatibility for scrapy.conf.settings singleton ---
 6     if settings is None and 'scrapy.conf' in sys.modules:
 7         from scrapy import conf
 8         if hasattr(conf, 'settings'):
 9             settings = conf.settings
10     # ------------------------------------------------------------------
11 
12     if settings is None:
13         settings = get_project_settings()
14     check_deprecated_settings(settings)
15 
16     # --- backwards compatibility for scrapy.conf.settings singleton ---
17     import warnings
18     from scrapy.exceptions import ScrapyDeprecationWarning
19     with warnings.catch_warnings():
20         warnings.simplefilter("ignore", ScrapyDeprecationWarning)
21         from scrapy import conf
22         conf.settings = settings
23     # ------------------------------------------------------------------
24 
25     inproject = inside_project()
26     cmds = _get_commands_dict(settings, inproject)
27     cmdname = _pop_command_name(argv)
28     parser = optparse.OptionParser(formatter=optparse.TitledHelpFormatter(), 
29         conflict_handler='resolve')
30     if not cmdname:
31         _print_commands(settings, inproject)
32         sys.exit(0)
33     elif cmdname not in cmds:
34         _print_unknown_command(settings, cmdname, inproject)
35         sys.exit(2)
36 
37     cmd = cmds[cmdname]
38     parser.usage = "scrapy %s %s" % (cmdname, cmd.syntax())
39     parser.description = cmd.long_desc()
40     settings.setdict(cmd.default_settings, priority='command')
41     cmd.settings = settings
42     cmd.add_options(parser)
43     opts, args = parser.parse_args(args=argv[1:])
44     _run_print_help(parser, cmd.process_options, args, opts)
45 
46     cmd.crawler_process = CrawlerProcess(settings)
47     _run_print_help(parser, _run_command, cmd, args, opts)
48     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:

1 def _run_print_help(parser, func, *a, **kw):
2     try:
3         func(*a, **kw)
4     except UsageError as e:
5         if str(e):
6             parser.error(str(e))
7         if e.print_help:
8             parser.print_help()
9         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:

1 def _run_command(cmd, args, opts):
2     if opts.profile or opts.lsprof:
3         _run_command_profiled(cmd, args, opts)
4     else:
5         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:

 1 class ScrapyCommand(object):
 2 
 3     requires_project = False
 4     crawler_process = None
 5 
 6     # default settings to be used for this command instead of global defaults
 7     default_settings = {}
 8 
 9     exitcode = 0
10 
11     def __init__(self):
12         self.settings = None  # set in scrapy.cmdline
13 
14     def set_crawler(self, crawler):
15         assert not hasattr(self, '_crawler'), "crawler already set"
16         self._crawler = crawler
17 
18     @property
19     def crawler(self):
20         warnings.warn("Command's default `crawler` is deprecated and will be removed. "
21             "Use `create_crawler` method to instatiate crawlers.",
22             ScrapyDeprecationWarning)
23 
24         if not hasattr(self, '_crawler'):
25             crawler = self.crawler_process.create_crawler()
26 
27             old_start = crawler.start
28             self.crawler_process.started = False
29 
30             def wrapped_start():
31                 if self.crawler_process.started:
32                     old_start()
33                 else:
34                     self.crawler_process.started = True
35                     self.crawler_process.start()
36 
37             crawler.start = wrapped_start
38 
39             self.set_crawler(crawler)
40 
41         return self._crawler
42 
43     def syntax(self):
44 
45     def short_desc(self):
46 
47     def long_desc(self):
48 
49     def help(self):
50 
51     def add_options(self, parser):
52 
53     def process_options(self, args, opts):
54     
55     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