From 97980a8f94e0c3f868dc071a24c299d05b017de5 Mon Sep 17 00:00:00 2001 From: Todd Gamblin Date: Sat, 28 Sep 2019 20:59:02 -0700 Subject: [PATCH] prefer Python 3 to Python 2 for running Spack The Python landscape is going to be changing in 2020, and Python 2 will be end of life. Spack should *prefer* Python 3 to Python 2 by default, but we still need to run on systems that only have Python2 available. This is trickier than it sounds, as on some systems, the `python` command is `python2`; on others it's `python3`, and RHEL8 doesn't even have the `python` command. Instead, it makes you choose `python3` or `python2`. You can thus no longer make a simple shebang to handle all the cases. This commit makes the `spack` script bilingual. It is still valid Python, but its shebang is `#!/bin/sh`, and it has a tiny bit of shell code at the beginning to pick the right python and execute itself with what it finds. This has a lot of advantages. I think this will help ensure that Spack works well in Python3 -- there are cases where we've missed things because Python2 is still the default `python` on most systems. Also, with this change, you do not lose the ability to execute the `spack` script directly with a python interpreter. This is useful for forcing your own version of python, running coverage tools, and running profiling tools. i.e., these will not break with this change: ```console $ python2 $(which spack) $ coverage run $(which spack) $ pyinstrument $(which spack) ``` These would not work if we split `spack` into a python file and a shell script (see #11783). So, this gives us the best of both worlds. We get to control our interpreter *and* remain a mostly pure python executable. --- bin/spack | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/bin/spack b/bin/spack index 7c911ce9f2..a7ebb8d161 100755 --- a/bin/spack +++ b/bin/spack @@ -1,10 +1,26 @@ -#!/usr/bin/env python +#!/bin/sh +# -*- python -*- # # Copyright 2013-2019 Lawrence Livermore National Security, LLC and other # Spack Project Developers. See the top-level COPYRIGHT file for details. # # SPDX-License-Identifier: (Apache-2.0 OR MIT) +# This file is bilingual. The following shell code finds our preferred python. +# Following line is a shell no-op, and starts a multi-line Python comment. +# See https://stackoverflow.com/a/47886254 +""":" +# prefer python3, then python, then python2 +for cmd in python3 python python2; do + command -v > /dev/null $cmd && exec $cmd $0 "$@" +done + +echo "==> Error: spack could not find a python interpreter!" >&2 +exit 1 +":""" +# Line above is a shell no-op, and ends a python multi-line comment. +# The code above runs this file with our preferred python interpreter. + from __future__ import print_function import os