How does python urlopen use url, parameters, header and context? at the same time - Codes Helper

problem description

how does python urlopen use url, parameters, header information and context? at the same time
the original code looks like this:

req = urllib2.Request(url, obj, headers)
opener = urllib2.urlopen(req)

now there is an error in [SSL: CERTIFICATE_VERIFY_FAILED]

so I need to use the following code to handle the error report:

import ssl
import urllib2
context = ssl._create_unverified_context()
print urllib2.urlopen("https://aa.com/", context=context).read()

could you tell me how to add this context?
tried print urllib2.urlopen (url=req, context=context). Read ()
error report:
HTTP Error 503: Service Unavailable

)

I use the following code to report the same error HTTP Error 503: Service Unavailable

import ssl
ssl._create_default_https_context = ssl._create_unverified_context

debugging information:

Internal Server Error: /deploy/key_list_import/
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/lib/python2.7/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 23, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/project/soms/deploy/views.py", line 398, in salt_key_import
    minions,minions_pre = sapi.list_all_key()
  File "/project/soms/deploy/saltapi.py", line 54, in list_all_key
    self.token_id()
  File "/project/soms/deploy/saltapi.py", line 30, in token_id
    content = self.postRequest(obj,prefix="/login")
  File "/project/soms/deploy/saltapi.py", line 42, in postRequest
    opener = urllib2.urlopen(req, context=ssl._create_unverified_context())
  File "/usr/lib64/python2.7/urllib2.py", line 154, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib64/python2.7/urllib2.py", line 437, in open
    response = meth(req, response)
  File "/usr/lib64/python2.7/urllib2.py", line 550, in http_response
    "http", request, response, code, msg, hdrs)
  File "/usr/lib64/python2.7/urllib2.py", line 475, in error
    return self._call_chain(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 558, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
HTTPError: HTTP Error 503: Service Unavailable