There need not be any parallelism or simultaneity for a data race to occur. What matters is that writes occur concurrently with another read/write, not whether

There need not be any parallelism or simultaneity for a data race to occur. What matters is that writes occur concurrently with another read/write, not whether they occur in parallel. For example it's possible on a single core CPU to have one thread operate on the higher bits of a 32-bit memory location then yield to second thread that operates on the lower 32-bits of the same memory location so that the end result is a clobbered write. At no point do the two threads run simultaneously and yet this would still be an example of a data race. Python's GIL makes such an access impossible on any platform it runs on.