I've done a fair bit of testing and it seems rather inconsistent as to whether either of these work when added right before an explicit call to `os.fork()`:

os.environ['OBJC_DISABLE_INITIALIZE_FORK_SAFETY'] = 'YES'

ctyles.cdll.LoadLibrary('/System/Library/Frameworks/Foundation.framework/Foundation')

Neither of these seems to reliably prevent the __NSPlaceholderDate warning, nor prevent the core dumps.  The best I've been able to do is to prevent them by setting the environment variable *before* the parent process starts (i.e. outside of the Python code of the process).