21.10. "urllib.robotparser" --- robots.txt 语法分析程序
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**源代码：** Lib/urllib/robotparser.py

======================================================================

This module provides a single class, "RobotFileParser", which answers
questions about whether or not a particular user agent can fetch a URL
on the Web site that published the "robots.txt" file.  For more
details on the structure of "robots.txt" files, see
http://www.robotstxt.org/orig.html.

class urllib.robotparser.RobotFileParser(url='')

   This class provides methods to read, parse and answer questions
   about the "robots.txt" file at *url*.

   set_url(url)

      Sets the URL referring to a "robots.txt" file.

   read()

      Reads the "robots.txt" URL and feeds it to the parser.

   parse(lines)

      Parses the lines argument.

   can_fetch(useragent, url)

      Returns "True" if the *useragent* is allowed to fetch the *url*
      according to the rules contained in the parsed "robots.txt"
      file.

   mtime()

      Returns the time the "robots.txt" file was last fetched.  This
      is useful for long-running web spiders that need to check for
      new "robots.txt" files periodically.

   modified()

      Sets the time the "robots.txt" file was last fetched to the
      current time.

   crawl_delay(useragent)

      Returns the value of the "Crawl-delay" parameter from
      "robots.txt" for the *useragent* in question.  If there is no
      such parameter or it doesn't apply to the *useragent* specified
      or the "robots.txt" entry for this parameter has invalid syntax,
      return "None".

      3.6 新版功能.

   request_rate(useragent)

      Returns the contents of the "Request-rate" parameter from
      "robots.txt" as a *named tuple* "RequestRate(requests,
      seconds)". If there is no such parameter or it doesn't apply to
      the *useragent* specified or the "robots.txt" entry for this
      parameter has invalid syntax, return "None".

      3.6 新版功能.

The following example demonstrates basic use of the "RobotFileParser"
class:

   >>> import urllib.robotparser
   >>> rp = urllib.robotparser.RobotFileParser()
   >>> rp.set_url("http://www.musi-cal.com/robots.txt")
   >>> rp.read()
   >>> rrate = rp.request_rate("*")
   >>> rrate.requests
   3
   >>> rrate.seconds
   20
   >>> rp.crawl_delay("*")
   6
   >>> rp.can_fetch("*", "http://www.musi-cal.com/cgi-bin/search?city=San+Francisco")
   False
   >>> rp.can_fetch("*", "http://www.musi-cal.com/")
   True
