Here are the two test cases: the one with FileIO.readall() fails

def testReadWithWritingMode(self):
    r, w = os.pipe()
    w = os.fdopen(w, "w")
    w.write("hello")
    w.close()
    with io.FileIO(r, mode="w") as f:
        with self.assertRaises(_io.UnsupportedOperation):
            f.read()

def testReadallWithWritingMode(self):
    r, w = os.pipe()
    w = os.fdopen(w, "w")
    w.write("hello")
    w.close()
    with io.FileIO(r, mode="w") as f:
        with self.assertRaises(_io.UnsupportedOperation):
            f.readall()

With FileIO.read() raises "UnsupportedOperation" in "w" mode, I expect FileIO.readall() do the same. But in fact FileIO.readall() does not check for readable and does not raise "UnsupportedOperation" in "w"mode.

I'm happy to write a pull request if you want.