== The right way for your WSGI app to know if it's using HTTPS

Suppose that you have a WSGI application that's running under Apache,
either directly as a CGI-BIN through [[some lashup
ExploitingPolymorphicWSGI]] or perhaps through an (old) version of
``mod_wsgi'' (such as Django on an Ubuntu 12.04 host, which has
``mod_wsgi'' version 3.3). Suppose that you want to know if you're
being invoked via a HTTPS URL, either for security purposes or for
your own internal reasons (for example, you might need separate
[[page caches ../web/DynamicSiteCaching]] for HTTP versus HTTPS
requests). What is the correct way to do this?

If you're me, for a long time you do the obvious thing; you look
at the _HTTPS_ environment variable that your WSGI application
inherits from Apache (or the web server of your choice, if you're
also running things under [[an alterate ../web/AbandoningLighttpd]]).
If it has the value _on_ or sometimes _1_, you've got a HTTPS
connection; if it doesn't exist or has some other value, you don't.

As I learned recently by reading [[some ``mod_wsgi'' release notes
https://modwsgi.readthedocs.org/en/latest/release-notes/version-4.1.0.html]],
this is in practice wrong (and probably wrong even in theory). What
I should be doing is checking ((wsgi.url_scheme)) from the (WSGI)
environment to see if it was _"https"_ or _"http"_. Newer versions
of ``mod_wsgi'' explicitly strip the _HTTPS_ environment variable
and anyways, as [[the WSGI PEP https://www.python.org/dev/peps/pep-3333/]]
makes clear, including a _HTTPS_ environment variable was always a
'maybe' thing.

(You can argue that ``mod_wsgi'' is violating the spirit of the
'should' in the PEP here, but I'm sure it has its reasons for this
particular change.)

Not using ((wsgi.url_scheme)) was always kind of conveniently lazy;
I was pretending that WSGI was still basically a CGI-BIN environment
when it's not really. I always should have been preferring _wsgi._
environment variables where they were available, and ((wsgi.url_scheme))
has always been there. But I change habits slowly when nothing
smacks me over the nose about them.

(This may have been part of an ``mod_wsgi'' issue I ran into at one
point, but that's [[another entry ModWsgiDualSchemaProblem]].)