See https://lwn.net/Articles/606141/ for an explanation of the blocking behavior of getrandom(). This makes sense to me - before the pool has initialized, /dev/urandom will be readable but will return highly predictable data - ie: it should not be considered safe. In other words, I think that getrandom() offers a sensible API.

The only circumstances where we hit the EAGAIN in getrandom() should be when it's called extremely early in the boot process (as is the case for the systemd-cron generator script I mentioned earlier). I think this is safe enough; a more thorough approach would be to flag that the per-process hash seed (_Py_HashSecret) is predictable and shouldn't be used.