If you use Apache (as we do) and have relatively casual WSGI-based applications (again, as we do), then Apache's mod_wsgi is often the easiest way to deploy your WSGI application. Speaking as a system administrator, it's quite appealing to not have to manage a separate configuration and a separate daemon (and I still get process separation and different UIDs). But at the moment there is a little problem, at least for people (like us) who use their Unix distribution's provided version of Apache and mod_wsgi rather than build your own. The problem is that any given build of mod_wsgi only supports one version of (C)Python.

(Mod_wsgi contains an embedded CPython interpreter, although generally it's not literally embedded; instead mod_wsgi is linked to the appropriate libpython shared library.)

In the glorious future there will only be (some version of) Python 3, and this will not be an issue. All of your WSGI programs will be Python 3, mod_wsgi will use some version of Python 3, and everything will be relatively harmonious. In the current world, there is still a mixture of Python 2 and Python 3, and if you want to run a WSGI based program written in a different version of Python than your mod_wsgi supports, you will be sad. As a corollary of this, you just can't run both Python 2 and Python 3 WSGI applications under mod_wsgi in a single Apache.

Some distributions have both Python 2 and Python 3 versions of mod_wsgi available; this is the case for Ubuntu 20.04 (which answers something I wondered about last January). This at least lets you pick whether you're going to run Python 2 or Python 3 WSGI applications on any given system. Hopefully no current Unix restricts itself to only a Python 2 mod_wsgi, since there's an increasing number of WSGI frameworks that only run under Python 3.

(For example, Django last supported Python 2 in 1.11 LTS, which is no longer supported; support stopped some time last year.)

PS: Since I just looked it up, CentOS 7 has a Python 3 version of mod_wsgi in EPEL, and Ubuntu 18.04 has a Python 3 version in the standard repositories.