Python overrides OpenSSL's default cipher suites because the default selection used to be pretty bad and insecure. Python disables cipher suites with insecure algorithm such as RC4, MD5, DES, and 3DES. The SSL module has hard-coded cipher strings for SSLContext and ssl.create_default_context() in multiple places:

* https://github.com/python/cpython/blob/v3.6.2/Modules/_ssl.c#L2693
* https://github.com/python/cpython/blob/v3.6.2/Lib/ssl.py#L387
* https://github.com/python/cpython/blob/v3.6.2/Lib/ssl.py#L503

However the cipher suite overrides makes it harder for vendors and downstream to enforce consistent policies. For example the hard-coded strings disable Fedora's crypto policy, https://fedoraproject.org/wiki/Changes/CryptoPolicy . Fedora has patched OpenSSL to support a "PROFILE=SYSTEM" cipher suite string. The string causes OpenSSL to read crypto settings from a system wide configuration file.

In order to make it easier to override the default string, Python should have a configure option --with-ssl-default-suite that defines a PY_SSL_DEFAULT_SUITE macro. In the absence of the option / macro, Python shall set a sensible default suite. Application are encouraged to use this default suite list. They are still free to override the default by calling SSLContext's set_ciphers() method.

Fedora's OpenSSL patch: https://src.fedoraproject.org/cgit/rpms/openssl.git/tree/openssl-1.1.0-system-cipherlist.patch?h=f26

My PR is ready for review.

New changeset 892d66e422d5367673163d62ba40cd70a37d5cf7 by Christian Heimes in branch 'master':
bpo-31429: Define TLS cipher suite on build time (#3532)
https://github.com/python/cpython/commit/892d66e422d5367673163d62ba40cd70a37d5cf7