[Python-Dev] Clarification of PEP 394 for scripts that run under Python 2 and 3

[Damien George]

Hi python-dev,

We have a Python script that runs correctly under Python 2.6, 2.7 and 3.3+.
It is executed on a *nix system using the "python" executable (ie not
python2 or python3 specifically). This works just fine for systems that
have Python 2 installed, or 2 and 3, or just 3 and symlink "python" to
"python3" (eg Arch Linux).

But it fails for systems that have only Python 3 and do not create a
"python" symlink, ie only "python3" exists as an executable.

We thought that PEP 394 would come to the rescue here but it seems to be
unclear on this point. In particular it says:

- 4th point of the abstract: "so python should be used in the shebang line
only for scripts that are source compatible with both Python 2 and 3"

- 6th point of the recommendation section: "One exception to this is
scripts that are deliberately written to be source compatible with both
Python 2.x and 3.x. Such scripts may continue to use python on their
shebang line without affecting their portability"

- 8th point in the migration notes: "If these conventions are adhered to,
it will become the case that the python command is only executed in an
interactive manner as a user convenience, or to run scripts that are source
compatible with both Python 2 and Python 3."

Well, that's pretty clear to me: one can expect the "python" executable to
be available to run scripts that are compatible with versions 2.x and 3.x.

The confusion comes because there are systems that install Python 3 without
creating a "python" symlink (hence breaking the above).  And Guido said
that "'python' should always be the same as 'python2'" (see
https://mail.python.org/pipermail/python-dev/2014-September/136389.html).
Further, Nick Coghlan seemed to agree that "when there's only python3
installed, there should be no /usr/bin/python" (see
https://mail.python.org/pipermail/python-dev/2014-September/136527.html).

My questions are:

1. What is the true intent of PEP 394 when only Python 3 is installed?  Is
"python" available or not to run scripts compatible with 2.x and 3.x?

2. Is it possible to write a shebang line that supports all variations of
Python installations on *nix machines?

3. If the answer to 2 is no, then what is the recommended way to support
all Python installations with one standalone script?

Thanks!

Regards,
Damien.
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