13.3. "robotparser" —  Parser for robots.txt
********************************************

注解: The "robotparser" module has been renamed "urllib.robotparser"
  in Python 3. The *2to3* tool will automatically adapt imports when
  converting your sources to Python 3.

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 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.

The following example demonstrates basic use of the RobotFileParser
class.

   >>> import robotparser
   >>> rp = robotparser.RobotFileParser()
   >>> rp.set_url("http://www.musi-cal.com/robots.txt")
   >>> rp.read()
   >>> rp.can_fetch("*", "http://www.musi-cal.com/cgi-bin/search?city=San+Francisco")
   False
   >>> rp.can_fetch("*", "http://www.musi-cal.com/")
   True
