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
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 = re.sub(r'(-script\.pyw|\.exe)?$', '', sys.argv)
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
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)
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
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