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For example, Jython 2.7. When we try to execute `jython-standalone -m pip`, distutils.sysconfig tries to parse sys.executable as a path and obviously fails:
 Traceback (most recent call last):
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/runpy.py", line 161, in _run_module_as_main
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/runpy.py", line 72, in _run_code
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/ensurepip/__main__.py", line 4, in <module>
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/ensurepip/__init__.py", line 220, in _main
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/ensurepip/__init__.py", line 123, in bootstrap
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/ensurepip/__init__.py", line 45, in _run_pip
 File "/tmp/tmp9QnVEm/pip-1.6-py2.py3-none-any.whl/pip/__init__.py", line 10, in <module>
 File "/tmp/tmp9QnVEm/pip-1.6-py2.py3-none-any.whl/pip/util.py", line 13, in <module>
 File "/tmp/tmp9QnVEm/pip-1.6-py2.py3-none-any.whl/pip/backwardcompat/__init__.py", line 115, in <module>
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/distutils/sysconfig.py", line 28, in <module>
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/posixpath.py", line 367, in realpath
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/posixpath.py", line 373, in _joinrealpath
 File "/home/ale/dev/3toj/jython-standalone-2.7.0.jar/Lib/posixpath.py", line 61, in isabs
 AttributeError: 'NoneType' object has no attribute 'startswith'

I'm not sure it is reasonable to expect distutils to work if sys.executable is not valid. What do you think distutils could do instead?

Well, project_base (which is what sys.executable is used for) is used only during build (i. e. python_build is True), if I understand it correctly. Normally, sys.prefix and sys.exec_prefix are used for all the paths.
Also, project_base can be explicitly set using _PYTHON_PROJECT_BASE environ variable, in which case we still don't need sys.executable (but it still would fail without one right now).

Yes, you are right; I was thinking that distutils and/or pip re-executed python for certain tasks, but upon reflection I don't think they do.

We re-execute Python to run setup.py. Even from wheels we do it to compile pyc files. 
Sent from my iPhone
> On Oct 29, 2016, at 2:38 PM, R. David Murray <report@bugs.python.org> wrote:
> 
> 
> R. David Murray added the comment:
> 
> Yes, you are right; I was thinking that distutils and/or pip re-executed python for certain tasks, but upon reflection I don't think they do.
> 
> ----------
> stage: -> needs patch
> 
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue28552>
> _______________________________________

This issue can be reproduced with:
diff --git a/Lib/site.py b/Lib/site.py
index ad1146332b..c850109c19 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -638,3 +638,5 @@ def _script():
 
 if __name__ == '__main__':
 _script()
+
+sys.executable = None
Attached PR 12875 fix distutils.sysconfig and the distutils build command if sys.executable is None or an empty string. I don't expect that everything works magically, but at least, it's possible to run "./python -m distutils.sysconfig" and use "make" in Python which runs "./python -E ./setup.py build". I'm surprised, but setup.py is able to build C extensions using sys.executable = None :-)
I made a similar fix for sysconfig in bpo-7774: commit 171ba0504aa778d81346ea56fc9000b29d4d3e1d.

New changeset 0ef8c157e9195df0115c54ba875a5efb92ac22fb by Victor Stinner in branch 'master':
bpo-28552: Fix distutils.sysconfig for empty sys.executable (GH-12875)
https://github.com/python/cpython/commit/0ef8c157e9195df0115c54ba875a5efb92ac22fb

New changeset 3076a3e0d1c54a2a6cc54c84521cd0f640d7cffb by Victor Stinner (Miss Islington (bot)) in branch '3.7':
bpo-28552: Fix distutils.sysconfig for empty sys.executable (GH-12875) (GH-12948)
https://github.com/python/cpython/commit/3076a3e0d1c54a2a6cc54c84521cd0f640d7cffb

New changeset f4edd39017a211d4544570a1e2ac2110ef8e51b4 by Victor Stinner in branch '2.7':
bpo-28552: Fix distutils.sysconfig for empty sys.executable (GH-12875) (GH-12949)
https://github.com/python/cpython/commit/f4edd39017a211d4544570a1e2ac2110ef8e51b4

I fixed the bug in Python 2.7, 3.7 and master branch (future 3.8).
Thanks Alexander P for the bug report ;-)