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authorTodd Gamblin <tgamblin@llnl.gov>2019-09-28 20:59:02 -0700
committerTodd Gamblin <tgamblin@llnl.gov>2019-09-29 09:32:04 -0700
commit97980a8f94e0c3f868dc071a24c299d05b017de5 (patch)
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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) <args> $ coverage run $(which spack) <args> $ pyinstrument $(which spack) <args> ``` 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.
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