> Another use of the deprecated unsafe preexec_fn was to call os.umask in the child prior to exec. What do you mean by "deprecated"? The parameter doesn't seem to be deprecated in the documentation: https://docs.python.org/dev/library/subprocess.html#subprocess.Popen And when I use it, it doesn't emit any warning: --- import os, subprocess, sys def func(): print(f"func called in {os.getpid()}") argv = [sys.executable, "-c", "pass"] print(f"parent: {os.getpid()}") subprocess.run(argv, preexec_fn=func) --- Output: --- $ ./python -Werror x.py parent: 22264 func called in 22265 --- If you want to deprecate it, it should be documented as deprecated and emit a DeprecatedWarning, no?