Re: [PATCH] AM_PATH_PYTHON: try "python2" et al. first

[Jack Kelly]

Kerrick Staley <address@hidden> writes:
> Usually, when AM_PATH_PYTHON is invoked with an argument of the form
> 2.X, it means that only a Python 2 interpreter should be used, and not
> a Python 3 interpreter. However, the "python" command invokes a Python
> 3 interpreter on some systems (see [1]), so if we try "python" first,
> we may get a Python 3 interpreter when really a Python 2 interpreter is
> needed. By changing the precedence so that the "python2(.X)" commands
> are tried first, a Python 2 interpreter will always be used if one is
> available (and of sufficiently high version).
>
> [1] http://www.python.org/dev/peps/pep-0394/

I disagree with this patch. The "Future Changes to this Recommendation"
section of PEP 394 anticipates changing the `python' symlink to
`python3' at some point in the future.

I think a better patch would change the search list based on the major
version passed as the first argument to AM_PATH_PYTHON:
a) If it's 2.x, search `python2 python2.7 python2.6 ... python'.
b) If it's 3.x, search `python3 python3.3 python3.2 python3.1 ... python'.
c) If nothing's given, search `python python2 python2.7 ... python2.0
   python3 python3.2 ... python3.0.'

In case (c), swap the search order to search python3 interpreters first
once PEP 394 updates its recommendation to make python symlink to a
python 3 interpreter.

Question: should this dispatch be done at `autoconf' time (via m4) or at
`./configure' time (via sh)? If a package wants to support python 2 and
3, I think they should inspect $PYTHON after a call to AM_PATH_PYTHON,
and not pass a shell variable as the first argument. Is there an
automake convention about which arguments can safely hold shell
variables?

-- Jack