multithreading - Why is a Python I/O bound task not blocked by the GIL?

Question

The python threading documentation states that "...threading is still an appropriate model if you want to run multiple I/O-bound tasks simultaneously", apparently because I/O-bound processes can avoid the GIL that prevents threads from concurrent execution in CPU-bound tasks.

But what I dont understand is that an I/O task still uses the CPU. So how could it not encounter the same issues? Is it because the I/O bound task will not require memory management?

Answer 1

All of Python's blocking I/O primitives release the GIL while waiting for the I/O block to resolve -- it's as simple as that! They will of course need to acquire the GIL again before going on to execute further Python code, but for the long-in-terms-of-machine-cycles intervals in which they're just waiting for some I/O syscall, they don't need the GIL, so they don't hold on to it!