I now understand why my script fails, and it is caused by this issue.

The sysconfig.py code has another major issue: the use of os.putenv. This changes the environment, without changing os.environ. The use of os.putenv should be replaced by setting keys in os.environ to make it easier to discover that changes have been made.

Even that is no good: setting the environment variable should only be done in distutils to ensure that the right build environment is used. It should not be set globally where it will affect code that it was never intended to affect.


BTW. Sridhar: could this be the reason you cannot find the correct reproduction steps for this?  Do you use a build script that is writting in python and run with a copy of python where sysconfig.get_config_var('MACOSX_DEPLOYMENT_TARGET') returns '10.3'?