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build: better support for python3 systems #14737

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build: better support for python3 systems 
Improve support for systems where `python` is actually `python3`.

Not all systems have a `python2` binary, so simply updating the shebang
won't work.

What we can do is apply some cleverness: start life as a shell script,
locate the python binary, then re-execute the script but this time as
python code.

Special care is taken to ensure that spaces in arguments are passed on
verbatim.
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bnoordhuis committed Aug 17, 2017
commit e1a700dcdcb3b69fb400bbec30de3cafba006269
13 changes: 12 additions & 1 deletion configure
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#!/usr/bin/env python
#!/bin/sh
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@gibfahn gibfahn Aug 12, 2017

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This will stop syntax highlighting from working. Not the end of the world I guess.


# Locate python2 interpreter and re-execute the script. Note that the
# mix of single and double quotes is intentional, as is the fact that
# the ] goes on a new line.
_=[ 'exec' '/bin/sh' '-c' '''
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@gibfahn gibfahn Aug 12, 2017 •
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Is changing $_ definitely okay in all shells? I know it's normally not user-modifiable, but IDK about csh and other arcane shells. I checked zsh, bash, and dash.

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@bnoordhuis bnoordhuis Aug 14, 2017

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/bin/sh is always a Bourne-compatible shell, so it should be okay.

which python2.7 >/dev/null && exec python2.7 "$0" "$@"
which python2 >/dev/null && exec python2 "$0" "$@"
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@gibfahn gibfahn Aug 12, 2017

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Maybe check for python2.6 as well?

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@bnoordhuis bnoordhuis Aug 14, 2017

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So, I deliberately didn't add python2.6 for two reasons:

  1. I wasn't sure if it should go before or after python2, and
  2. We supported python2.6 because of centos 5 but since we no longer support that platform...

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@gibfahn gibfahn Aug 14, 2017

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Okay, I guess dropping python2.6 should be a separate PR.

exec python "$0" "$@"
''' "$0" "$@"
]
del _

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@c9n c9n Jul 19, 2020 •
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@bnoordhuis I'm not very clear how it works. Could you explain a bit more about this block for me? Thanks.

_=[ 'exec' '/bin/sh' '-c' '''
which python2.7 >/dev/null && exec python2.7 "$0" "$@"
which python2 >/dev/null && exec python2 "$0" "$@"
exec python "$0" "$@"
''' "$0" "$@"
]
del _

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@bnoordhuis bnoordhuis Jul 19, 2020

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@c9n The _=[ ... ] is first evaluated as shell script that re-executes the whole script as python code when it finds the appropriate interpreter. As python code, it's effectively a no-op: it assigns to _ an array of strings that isn't otherwise used.

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@c9n c9n Jul 19, 2020

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@bnoordhuis Got it! Thank you so much.

import sys
if sys.version_info[0] != 2 or sys.version_info[1] not in (6, 7):
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