Hostname verification makes not much sense without verifying the certificate chain first. At the moment one has to set verify_mode to CERT_REQUIRED first:

>>> import ssl                                                                                                                                                                                                                                                                                                    
>>> ctx = ssl.SSLContext(ssl.PROTOCOL_TLS)
>>> ctx.check_hostname, ctx.verify_mode                                                                                                                                                                                                                                                                           
(False, <VerifyMode.CERT_NONE: 0>)

>>> ctx.check_hostname = True
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: check_hostname needs a SSL context with either CERT_OPTIONAL or CERT_REQUIRED
>>> ctx.verify_mode = ssl.CERT_REQUIRED
>>> ctx.check_hostname = True


On the other hand verify mode cannot be set to CERT_NONE without disabling check_hostname first. One has to remember to set the values in opposite order!

>>> ctx.verify_mode = ssl.CERT_NONE
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/heimes/dev/python/cpython/Lib/ssl.py", line 485, in verify_mode
    super(SSLContext, SSLContext).verify_mode.__set__(self, value)                                                                                                                                                                                                                                                
ValueError: Cannot set verify_mode to CERT_NONE when check_hostname is enabled.
>>> ctx.check_hostname = False
>>> ctx.verify_mode = ssl.CERT_NONE


I find this confusing. In order to support PROTOCOL_TLS_CLIENT with _create_unverified_context(), I had to modify the code to this abomination:

    if not check_hostname:
        context.check_hostname = False
    if cert_reqs is not None:
        context.verify_mode = cert_reqs
    if check_hostname:
        context.check_hostname = True

Rather than making our users to jump through additional hoops, check_hostname = True should just set CERT_REQUIRED. This is a sane and safe default. On the other hand, ssl.CERT_NONE shall *not* disable check_hostname and still fail with a ValueError if check_hostname is enabled.

By the way we should not suggest CERT_OPTIONAL here, too. For TLS client connections, CERT_OPTIONAL is not really optional. CERT_OPTIONAL: SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, verify_cb), CERT_REQUIRED: SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT, verify_cb). According to https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_verify.html SSL_VERIFY_FAIL_IF_NO_PEER_CERT is ignored in client mode. I'll open a new bug report.

New changeset e82c034496512139e9ea3f68ceda86c04bc7baab by Christian Heimes in branch 'master':
bpo-31431: SSLContext.check_hostname auto-sets CERT_REQUIRED (#3531)
https://github.com/python/cpython/commit/e82c034496512139e9ea3f68ceda86c04bc7baab