Serhiy: "(...) I think it is more correct to say that stdin is always unbuffered in Python 3."

I disagree. Technically, sys.stdin.read(1) reads up to 1024 bytes from the file descriptor 0. For me, "unbuffered read" means that read(1) reads a single byte.

Expected behaviour of an fully unbuffered stdin:

assert sys.stdin.read(1) == 'a'
assert os.read(0, 1) == b'b'

The program should not fail with an assertion error nor block if you write 'ab' characters into stdin.