TypeError: can't concat str to bytes

def download(url, user_agent='wswp', num_retries=2):
    print("Downloading: " + url)
    headers = {'User-agent': user_agent}
    req = request.Request(url, headers)
    try:
        html = request.urlopen(req).read()
    except request.URLError as e:
        print("Download error: " + e.reason)
        html = None
        if num_retries > 0:
            if hasattr(e, 'code') and 500 <= e.code < 600:
                return download(url, user_agent, num_retries - 1)
    return html

完整的报错信息

Traceback (most recent call last):
  File "E:/practice/demo.py", line 64, in <module>
    download('http://httpstat.us/500')
  File "E:/practice/demo.py", line 14, in download
    html = request.urlopen(req).read()
  File "E:\python\lib\urllib\request.py", line 223, in urlopen
    return opener.open(url, data, timeout)
  File "E:\python\lib\urllib\request.py", line 526, in open
    response = self._open(req, data)
  File "E:\python\lib\urllib\request.py", line 544, in _open
    '_open', req)
  File "E:\python\lib\urllib\request.py", line 504, in _call_chain
    result = func(*args)
  File "E:\python\lib\urllib\request.py", line 1346, in http_open
    return self.do_open(http.client.HTTPConnection, req)
  File "E:\python\lib\urllib\request.py", line 1318, in do_open
    encode_chunked=req.has_header('Transfer-encoding'))
  File "E:\python\lib\http\client.py", line 1239, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "E:\python\lib\http\client.py", line 1285, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "E:\python\lib\http\client.py", line 1234, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "E:\python\lib\http\client.py", line 1064, in _send_output
    + b'\r\n'
TypeError: can't concat str to bytes

就是request.Request()错误
将

req = request.Request(url, headers)

改为

req = request.Request(url=url,headers=headers)