Python scripts assume that "/usr/bin/env python" is python2

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Bugzilla Link	PR12900
Status	NEW
Importance	P enhancement
Reported by	Jed Brown (jed@59a2.org)
Reported on	2012-05-20 17:16:39 -0700
Last modified on	2013-09-29 06:13:17 -0700
Version	trunk
Hardware	PC Linux
CC	anton@korobeynikov.info, austinenglish@gmail.com, daniel@zuster.org, Deil.Christoph@gmail.com, e.bachmakov@gmail.com, geek4civic@gmail.com, llvm-bugs@lists.llvm.org, mtahmed@uwaterloo.ca, muhammad.t.ahmad@intel.com, samuel@oriontransfer.org, uzytkownik2@gmail.com
Fixed by commit(s)
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The build scripts are written in python2, but "/usr/bin/env python" is python3 on some systems (e.g. Arch Linux). There are at least two solutions

1. Add a --with-python=/path/to/python configure flag so the user can choose a compatible version. Then invoke the scripts using ${PYTHON} path/to/script.py.

2. Make the scripts hybrid python2/python3. This should be easy if you can assume python2.6 or later (to support importing print_function from future).

[Quuxplusone]

We have had some resistance in the past to moving to a newer Python than 2.4 (due to some BSDs), but honestly I think that moving to 2.6 is reasonably.

My preferred solution here would be to make our Python tools be hybrid python2/3.

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RHEL5 is stuck on Python 2.4 and supported until 2017. It has been the main limitation for projects that I'm involved in. But most installations I have encountered already needed a newer version of Python and have upgraded.

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I ran into this error trying to build ROOT (root.cern.ch, they now include a
copy of llvm in their SVN trunk):
https://savannah.cern.ch/bugs/?99139

What's the status on this one?
Any chance of fixing this quickly so that llvm 3.2 can be built with Python 3?

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Just for reference, here's the errors you'll get if the python on your PATH is
a Python 3.

Plenty of these ImportErrors, but the build keeps going:

llvm[4]: Constructing LLVMBuild project information.
Traceback (most recent call last):
  File "/Users/deil/software/llvm_Python3/llvm/utils/llvm-build/llvm-build", line 3, in <module>
    import llvmbuild
  File "/Users/deil/software/llvm_Python3/llvm/utils/llvm-build/llvmbuild/__init__.py", line 1, in <module>
    from main import main
ImportError: No module named main
llvm[4]: Compiling HexagonMCAsmInfo.cpp for Debug+Asserts build

Then after a few minutes the build fails like this:

llvm[2]: Building Debug+Asserts Archive Library libLLVMCodeGen.a
llvm[1]: Constructing LLVMBuild project information.
Traceback (most recent call last):
  File "/Users/deil/software/llvm_Python3/llvm/utils/llvm-build/llvm-build", line 3, in <module>
    import llvmbuild
  File "/Users/deil/software/llvm_Python3/llvm/utils/llvm-build/llvmbuild/__init__.py", line 1, in <module>
    from main import main
ImportError: No module named main
llvm[1]: Building llvm-config BuildVariables.inc file.
llvm[1]: Compiling llvm-config.cpp for Debug+Asserts build
/Users/deil/software/llvm_Python3/llvm/tools/llvm-config/llvm-config.cpp:45:10:
fatal error: 'LibraryDependencies.inc' file not found
#include "LibraryDependencies.inc"
         ^
1 error generated.
rm: /Users/deil/software/llvm_Python3/build/tools/llvm-
config/Debug+Asserts/llvm-config.d.tmp: No such file or directory
make[1]: *** [/Users/deil/software/llvm_Python3/build/tools/llvm-
config/Debug+Asserts/llvm-config.o] Error 1
make: *** [all] Error 1

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This is a major issue when building with MacPorts and Python3. It has been over a year, can we get some movement on this issue?

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According to http://www.python.org/dev/peps/pep-0394/ there should be a python2 executable everywhere python2 is installed?

So how about /usr/bin/env python -> /usr/bin/env python2 ?

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Note the date on the PEP. Recent distributions should have python2, but older installations (before 2010 or so) usually don't.

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At the very least, it would be good if there was a PYTHON environment variable that one could specify.

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(In reply to comment #8)
> At the very least, it would be good if there was a PYTHON environment
> variable that one could specify.

Not exactly an environment variable but there is CMake variable:

cmake ../llvm-3.3.src -DPYTHON_EXECUTABLE=/usr/bin/python2.7

(You can substitute /usr/bin/python2.7 by path that's working on your system).

At least quick search shown that FindPythonInterp allows to specified allow
versions (at least in newer CMake) but I haven't used CMake so I don't know how.