Thank you for the report. The issue is that the necessary operating system support for opening with a file descriptor, openat, was only first implemented in macOS 10.10 (see man 2 openat) and, up until recently, all python.org macOS binaries were built on 10.9 (or earlier) systems for compatibility across multiple operating system versions. As of Python 3.9.1, we now support "weak-linking", that is, building on newer systems but still able to run on older systems. The python.org macOS universal2 installer variant, introduced in 3.9.1, does support os.fwalk() on macOS 10.10+. (The legacy 10.9 Intel-64 only variant is still built on 10.9 but will be phased out by the release of Python 3.10.)  If you build recent Python versions from source with a deployment target of 10.10 or later, you should also find a working os.fwalk().