How to properly install PyROOT for Python 3?

[ksmith]

I am attempting to install the Python3 bindings for ROOT, but always seem to end up with Python2 bindings. I have used the CMake flags -Dpython=ON and -Dpython3=ON. I have both python2 and python3 installed and the python command defaults to the python 2 version. From a posting by @Graipher I have a suspicion that it has to do with what the default python version is (he has the opposite issue). Do I need to change where the default points before configuring to get the correct version?

PyROOT import error (PyInit_libPyROOT)

What makes this even more confusing (at least to me) is that if python points to python3 and you build it with python=ON and python3=OFF, it will use that default python and build it for Python 3…

Also mentioned here:

ROOT and python3

Before this I always manage to get ROOT install with python3, and always make sure that the libraries and include dir is for python3.6 however the latest version of ROOT 6.10, tried with 6.10/00, 6.10/02 and even 6.10/04, I could cmake and build it but I could not get root to work in python. All these on CentOS 7 For one things is I could not yum install libldap-dev and gsl-static as those package are not available Are those package required for python3 to build properly for root to work with…

And some useful information in a bug report here:
https://sft.its.cern.ch/jira/browse/ROOT-8260

The error message recieved when trying to start in python3 is:

$ python3.6
Python 3.6.2 (default, Sep 21 2017, 00:54:38) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.42)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ROOT
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/root/6.10.06-py3/lib/ROOT.py", line 24, in <module>
    import cppyy
  File "/opt/root/6.10.06-py3/lib/cppyy.py", line 61, in <module>
    import libPyROOT as _backend
ImportError: dynamic module does not define module export function (PyInit_libPyROOT)

Thanks

[ksmith]

I went ahead and tested this changing python to point to python3, rebuilt and I get the python3 bindings. Using this information and the info from @Graipher it seems the the -Dpython3 flag is ignored and the version is picked from what the alias python points to.

[ddavis]

You can use -DPYTHON_EXECUTABLE=/path/to/desired/python without aliasing anything.

[ksmith]

Ahh, useful. So the solution is to ignore the non-working python3 flag and use the PYTHON_EXECUTABLE flag.

Opened a bug report indicating that the python3 flag seems to be ignored: https://sft.its.cern.ch/jira/browse/ROOT-9033