while researching bpo-11191 I cam across 6 additional errors.

There is a test in Lib/test/support/init.py

def missing_compiler_executable(cmd_names=[]):
    """Check if the compiler components used to build the interpreter exist.

Check for the existence of the compiler executables whose names are listed
in 'cmd_names' or all the compiler executables when 'cmd_names' is empty
and return the first missing executable or None when none is found
missing.

"""
from distutils import ccompiler, sysconfig, spawn
compiler = ccompiler.new_compiler()
sysconfig.customize_compiler(compiler)
for name in compiler.executables:
    if cmd_names and name not in cmd_names:
        continue
    cmd = getattr(compiler, name)
    if cmd_names:
        assert cmd is not None, \
                "the '%s' executable is not configured" % name
    elif cmd is None:
        continue
    if spawn.find_executable(cmd[0]) is None:
        return cmd[0]

The "elif cmd is None:" is not successful because cmd maybe '' (null string)

Initially I thought to change to
"elif cmd is None or (not cmd):" but I hope I found a better resolution!

In: Lib/distutils/sysconfig.py the final bits of customize_compiler is:

        compiler.set_executables(
            preprocessor=cpp,
            compiler=cc_cmd,
            compiler_so=cc_cmd + ' ' + ccshared,
            compiler_cxx=cxx,
            linker_so=ldshared,
            linker_exe=cc,
            archiver=archiver)

the value for cxx come from os.environ, if set, otherwise it comes from get_sys_vars()

    (cc, cxx, opt, cflags, ccshared, ldshared, shlib_suffix, ar, ar_flags) = \
        get_config_vars('CC', 'CXX', 'OPT', 'CFLAGS',
                        'CCSHARED', 'LDSHARED', 'SHLIB_SUFFIX', 'AR', 'ARFLAGS')

If, during the build, CXX was not set - this sets cxx to the null string ('').

So the fix is to assign cxx = None when len(cxx) == 0

        if 'CXX' in os.environ:
            cxx = os.environ['CXX']
        if not len(cxx):
            cxx = None

While this only seems to happen for cxx - maybe this should be extended to all the variables in (cc, cxx, opt, cflags, ccshared, ldshared, shlib_suffix, ar, ar_flags)?

So, ultimately - I choose to go with changing : compiler.set_executables()

diff --git a/Lib/distutils/ccompiler.py b/Lib/distutils/ccompiler.py
index b71d1d39bc..2e08c4abd2 100644
--- a/Lib/distutils/ccompiler.py
+++ b/Lib/distutils/ccompiler.py
@@ -148,7 +148,7 @@ class CCompiler:
             if key not in self.executables:
                 raise ValueError("unknown executable '%s' for class %s" %
                       (key, self.__class__.__name__))
-            self.set_executable(key, kwargs[key])
+            self.set_executable(key, kwargs[key] if len(kwargs[key]) else None)

     def set_executable(self, key, value):
         if isinstance(value, str):

Was:
======================================================================
ERROR: test_run (distutils.tests.test_build_clib.BuildCLibTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/data/prj/python/git/python3-3.8/Lib/distutils/tests/test_build_clib.py", line 121, in test_run
    ccmd = missing_compiler_executable()
  File "/data/prj/python/git/python3-3.8/Lib/test/support/__init__.py", line 2730, in missing_compiler_executable
    if spawn.find_executable(cmd[0]) is None:
IndexError: list index out of range

======================================================================
ERROR: test_build_ext (distutils.tests.test_build_ext.BuildExtTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/data/prj/python/git/python3-3.8/Lib/distutils/tests/test_build_ext.py", line 62, in test_build_ext
    cmd = support.missing_compiler_executable()
  File "/data/prj/python/git/python3-3.8/Lib/test/support/__init__.py", line 2730, in missing_compiler_executable
    if spawn.find_executable(cmd[0]) is None:
IndexError: list index out of range

======================================================================
ERROR: test_get_outputs (distutils.tests.test_build_ext.BuildExtTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/data/prj/python/git/python3-3.8/Lib/distutils/tests/test_build_ext.py", line 308, in test_get_outputs
    cmd = support.missing_compiler_executable()
  File "/data/prj/python/git/python3-3.8/Lib/test/support/__init__.py", line 2730, in missing_compiler_executable
    if spawn.find_executable(cmd[0]) is None:
IndexError: list index out of range

======================================================================
ERROR: test_build_ext (distutils.tests.test_build_ext.ParallelBuildExtTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/data/prj/python/git/python3-3.8/Lib/distutils/tests/test_build_ext.py", line 62, in test_build_ext
    cmd = support.missing_compiler_executable()
  File "/data/prj/python/git/python3-3.8/Lib/test/support/__init__.py", line 2730, in missing_compiler_executable
    if spawn.find_executable(cmd[0]) is None:
IndexError: list index out of range

======================================================================
ERROR: test_get_outputs (distutils.tests.test_build_ext.ParallelBuildExtTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/data/prj/python/git/python3-3.8/Lib/distutils/tests/test_build_ext.py", line 308, in test_get_outputs
    cmd = support.missing_compiler_executable()
  File "/data/prj/python/git/python3-3.8/Lib/test/support/__init__.py", line 2730, in missing_compiler_executable
    if spawn.find_executable(cmd[0]) is None:
IndexError: list index out of range

======================================================================
ERROR: test_record_extensions (distutils.tests.test_install.InstallTestCase)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/data/prj/python/git/python3-3.8/Lib/distutils/tests/test_install.py", line 200, in test_record_extensions
    cmd = test_support.missing_compiler_executable()
  File "/data/prj/python/git/python3-3.8/Lib/test/support/__init__.py", line 2730, in missing_compiler_executable
    if spawn.find_executable(cmd[0]) is None:
IndexError: list index out of range

I'm not sure that the resolution currently suggested, changing compiler.set_executables(), is the right way to go.

This change to distutils is a break of backwards compatibility. Though it is a minor change, it could still break existing code.

Fixing test.support seems just as good to me in terms of code design, and better in that it is only used internally for our tests.

(BTW, instead of `elif cmd is None or (not cmd):`, you can just use `elif not cmd:`.)

On 10/26/2018 5:36 PM, Tal Einat wrote:
Tal Einat <taleinat@gmail.com> added the comment:

I'm not sure that the resolution currently suggested, changing compiler.set_executables(), is the right way to go.

This change to distutils is a break of backwards compatibility. Though it is a minor change, it could still break existing code.

Fixing test.support seems just as good to me in terms of code design, and better in that it is only used internally for our tests.

(BTW, instead of `elif cmd is None or (not cmd):`, you can just use `elif not cmd:`.)
Thx. One of the python bits I need to embrace.

Although - during my testing I saw that the null string "" was not being
accepted as None, but was accepted as "not cmd".

My reading error was taking None to mean a value was not initialized
while it seems to be None is a 'value' that is assigned, while ´not xxx'
can be either - never assigned or a null-length string.

This is the "ambiguity", at least for myself, that I feel I tracked down.

It is "unfortunate" if this breaks backward compatibility - IF the
backward compatibility has been based on an ambiguity. Or I again, miss
a fine-point of Python coding. Won´'t be the last time I expect.

Comments requested.

----------
nosy: +taleinat

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue34897\>

---

not cmd will be true if cmd is None, so it is completely equivalent to cmd is None or not cmd. This is a purely stylistic change which doesn't alter the logic.

To get a clear understanding of what's going on, I recommend reading the short "Truth Value Testing" section in the docs:
https://docs.python.org/library/stdtypes.html#truth-value-testing

Thx.

So, while "" is not None (i.e., "" is not False), it does test as a Boolean expression as 'False' so the test for None or "" is the same as testing for "" (but not the same as testing for None).

I accept your logic - and shall make the change in the test in Lib/test/support/init.py rather than in the assignment in Lib/distutils/ccompiler.py

Michael, please read more about comparisons[1] and the None object[2].

[1] https://docs.python.org/3/library/stdtypes.html#comparisons
[2] https://docs.python.org/3/library/stdtypes.html#the-null-object

New changeset 259c159 by Nick Coghlan (Michael Felt) in branch 'master':
bpo-34897: avoid distutils test error when CXX is not set (GH-9706)
259c159

This issue also still affects Python versions 3.6.15 and 3.7.12

IMHO it would make sense to backport this patch to the 3.6 and 3.7 branches, especially as it only affects one line of code and doesn't seem to affect anything else, but solves the same issue for prior python versions as the merged patch did for 3.8+