先贴一下原来的代码，是按照书上直接抄下来的

from urllib.robotparser import RobotFileParser
from urllib.request import urlopen
rp = RobotFileParser()
rp.parse(urlopen('//www.greatytc.com/robots.txt').read().decode('utf-8').split('\n'))
print(rp.can_fetch('*', '//www.greatytc.com/p/b67554025d7d'))
print(rp.can_fetch('*', '//www.greatytc.com/search?q=python&page=1&type=collections'))

然后就Pycharm就报出了如下错误：

Traceback (most recent call last):
  File "E:/PythonProject/PaChong/first.py", line 15, in <module>
    rp.parse((urlopen('//www.greatytc.com/robots.txt').read().decode('utf-8').split('\n')))
  File "E:\Python\lib\urllib\request.py", line 222, in urlopen
    return opener.open(url, data, timeout)
  File "E:\Python\lib\urllib\request.py", line 531, in open
    response = meth(req, response)
  File "E:\Python\lib\urllib\request.py", line 641, in http_response
    'http', request, response, code, msg, hdrs)
  File "E:\Python\lib\urllib\request.py", line 563, in error
    result = self._call_chain(*args)
  File "E:\Python\lib\urllib\request.py", line 503, in _call_chain
    result = func(*args)
  File "E:\Python\lib\urllib\request.py", line 755, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "E:\Python\lib\urllib\request.py", line 531, in open
    response = meth(req, response)
  File "E:\Python\lib\urllib\request.py", line 641, in http_response
    'http', request, response, code, msg, hdrs)
  File "E:\Python\lib\urllib\request.py", line 569, in error
    return self._call_chain(*args)
  File "E:\Python\lib\urllib\request.py", line 503, in _call_chain
    result = func(*args)
  File "E:\Python\lib\urllib\request.py", line 649, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 403: Forbidden

直接看最后一行，是HTTP Error 403:Frobidden
经过搜索，出现这个原因是因为用urllib.request.urlopen方式打开一个URL的话，服务器只会收到一个单纯的对于该页面访问的请求，但是服务器并不知道发送这个请求使用的浏览器，操作系统等信息，而缺失这些信息的访问往往都是非正常访问，会被一些网站禁止掉。
那么该如何解决这个问题呢？只需要在请求中加入UserAgent信息就行了
如下

from urllib.robotparser import RobotFileParser
from urllib import request
rp = RobotFileParser()
headers = {
    'User-Agent': 'Mozilla/4.0(compatible; MSIE 5.5; Windows NT)'
}
url = '//www.greatytc.com/robots.txt'
req = request.Request(url=url, headers=headers)
response = request.urlopen(req)
rp.parse(response.read().decode('utf-8').split('\n'))
print(rp.can_fetch('*', '//www.greatytc.com/p/b67554025d7d'))
print(rp.can_fetch('*', '//www.greatytc.com/search?q=python&page=1&type=collections'))

这样问题就完美解决了