The following test program:

import sys
try:
    import urllib.request as urllib_request
except:
    import urllib2 as urllib_request

print(sys.version)

handler = urllib_request.HTTPSHandler(debuglevel=1)
opener = urllib_request.build_opener(handler)
print(opener.open('https://httpbin.org/user-agent').read().decode('utf-8'))

Works as expected in Python 2:
$ python2 test_urllib_debuglevel.py
2.7.11 (default, Mar 31 2016, 06:18:34) 
[GCC 5.3.0]
send: 'GET /user-agent HTTP/1.1\r\nAccept-Encoding: identity\r\nHost: httpbin.org\r\nConnection: close\r\nUser-Agent: Python-urllib/2.7\r\n\r\n'
reply: 'HTTP/1.1 200 OK\r\n'
header: Server: nginx
header: Date: Sat, 30 Apr 2016 10:02:32 GMT
header: Content-Type: application/json
header: Content-Length: 40
header: Connection: close
header: Access-Control-Allow-Origin: *
header: Access-Control-Allow-Credentials: true
{
  "user-agent": "Python-urllib/2.7"
}

But the verbose output is unavailable in Python 3.x:
$ ./python test_urllib_debuglevel.py
3.6.0a0 (default:1ceb91974dc4, Apr 30 2016, 17:44:57) 
[GCC 5.3.0]
{
  "user-agent": "Python-urllib/3.6"
}

Patch attached.

Hi Chi Hsuan,

The patch looks good to me. Additional tests for the coverage of this will be helpful too. Thanks!

Patch for the test.

New changeset 63e23b2dedcc by Senthil Kumaran in branch '3.5':
Issue #26892: Honor debuglevel flag in urllib.request.HTTPHandler.
https://hg.python.org/cpython/rev/63e23b2dedcc

New changeset e0ec68d46464 by Senthil Kumaran in branch 'default':
merge from 3.5
https://hg.python.org/cpython/rev/e0ec68d46464

Thank you for the patch, Chi Hsuan Yen.