Restore Python 2.6+ compatibility

[vbraun]

As of Sage 7.3.beta6, building Sage requires a system Python >= 2.7 because sage-uncompress-spkg uses argparse.

While we already ship a copy of argparse.py, sage-uncompress-spkg currently doesn't try to import it from where it's located in the Sage tree.

Depends on #19984
Depends on #20871

CC: @dimpase @embray @jdemeyer

Component: build

Keywords: argparse sage-uncompress-spkg

Author: Volker Braun

Branch/Commit: 775a3d3

Reviewer: Erik Bray, Leif Leonhardy, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/20802

[vbraun]

Branch: u/vbraun/sage_requires_python_2_7_

[vbraun]

New commits:

c323a93

Sage requires Python 2.7+

[vbraun]

Commit: c323a93

[vbraun]

Author: Volker Braun

[jhpalmieri]

comment:3

Just a few months ago, people were complaining about a change (fixed in #20252) which prevented Sage from building with Python 2.6. So I think this is a dangerous change to implement: we will lose some of our user base. See discussion in sage-devel. What is the motivation?

[vbraun]

comment:4

The fix has been broken in #20721

I've advocated bundling argparse (which is imho a no-brainer) but the 0.1% size increase was deemed unacceptable in #19984.

[embray]

comment:5

Is use of argparse the only thing that currently breaks compatibility for Python 2.6? If so I think bundling it is a no-brainer--I've done it in a dozen other projects (though most projects are finally droppping support for Python 2.6).

That said we either support Python 2.6 or we don't. If Sage isn't tested against Python 2.6 it doesn't support it as far as anyone should be concerned. Same goes for any other Python version. If it's not tested it's not supported.

[dimpase]

comment:6

Replying to @embray:

Is use of argparse the only thing that currently breaks compatibility for Python 2.6? If so I think bundling it is a no-brainer--I've done it in a dozen other projects (though most projects are finally droppping support for Python 2.6).

That said we either support Python 2.6 or we don't. If Sage isn't tested against Python 2.6 it doesn't support it as far as anyone should be concerned. Same goes for any other Python version. If it's not tested it's not supported.

there is demand for Python 2.6 support. It should be easy to set up a VM with one of these LTS Linux systems which are using 2.6 as the default. The question is who will do this, and on what system. There is spare capacity on the UW cluster, I think, to do this.

[dimpase]

comment:7

So, what should be done to reenable 2.6?

[embray]

comment:8

I guess also I was confused about using Python as a build dependency versus as a runtime dependency. As a runtime dependency only Python 2.7 is supported I guess. But for the build tools 2.6+ makes sense (but should be tested if we're to claim to support it).