Deprecate and eventually remove macOS-only PYTHONEXECUTABLE environ variable

[ned-deily]

BPO	46901
Nosy	@ronaldoussoren, @ned-deily, @zooba

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-02.17:45:33.785>
labels = ['OS-mac', '3.11', 'docs']
title = 'Deprecate and eventually remove macOS-only PYTHONEXECUTABLE environ variable'
updated_at = <Date 2022-03-02.17:45:33.785>
user = 'https://github.com/ned-deily'

bugs.python.org fields:

activity = <Date 2022-03-02.17:45:33.785>
actor = 'ned.deily'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'macOS']
creation = <Date 2022-03-02.17:45:33.785>
creator = 'ned.deily'
dependencies = []
files = []
hgrepos = []
issue_num = 46901
keywords = []
message_count = 1.0
messages = ['414382']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'docs@python', 'steve.dower']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue46901'
versions = ['Python 3.11']

[ned-deily]

The PYTHONEXECUTABLE is a holdover from old Python 2 days; AFAIK, it was specifically to support the Build Applet tool which could be used to build macOS app bundles in Python 2. The only place in current Py3 releases where it is used is in the standard library is in the launch of IDLE.app, a usage that should be able to be eliminated. Otherwise, there shouldn't be any need for it by third-parties. For 3.11, we could eliminate its use by IDLE.app and add a deprecation warning to the docs; not sure about an execution time warning?

https://docs.python.org/3/using/cmdline.html#envvar-PYTHONEXECUTABLE

[rickeylev]

Hi,

I've found the PYTHONEXECUTABLE environment variable incredibly valuable for the particular case of relocatable virtual environments in read-only file systems using a wrapper script (not symlink) for the venv's bin/python. This case is common in the Bazel ecosystem, where as much as possible is created and determined at build time, and then copied over to a target system to actually run as-is.

In this case, what the PYTHONEXECUTABLE envvar essentially allows is allowing Python to find the underlying interpreter's home directory (which lets it find the interpreter's stdlib etc), while letting site.py's logic look at the venv's directory for e.g site-packages and sys.executable.

The net effect is you can copy a venv directory between systems without having to recreate pyvenv.cfg or the symlinks.

Also: PYTHONEEXECUTABLE is no-longer Mac specific. I'd have to go dig up issue/PR, but checking it only on Mac was intentionally dropped.