Robotparser doesn't support two quite important optional parameters from the robots.txt file. I have implemented those in the following way:
(Robotparser should be initialized in the usual way:
rp = robotparser.RobotFileParser()
rp.set_url(..)
rp.read
)

crawl_delay(useragent) - Returns time in seconds that you need to wait for crawling
if none is specified, or doesn't apply to this user agent, returns -1
request_rate(useragent) - Returns a list in the form [request,seconds].
if none is specified, or doesn't apply to this user agent, returns -1