def test_normal(self, capsys):
        """Test without changing sys.stderr/sys.stdout."""
        data = io.StringIO()
        with utils.fake_io(data.write):
            sys.stdout.write('hello\n')
            sys.stderr.write('world\n')

        out, err = capsys.readouterr()
        assert not out
        assert not err
        assert data.getvalue() == 'hello\nworld\n'

        sys.stdout.write('back to\n')
        sys.stderr.write('normal\n')
        out, err = capsys.readouterr()
        assert out == 'back to\n'
        assert err == 'normal\n'

    def test_stderr_replaced(self, capsys):
        """Test with replaced stderr."""
        data = io.StringIO()
        new_stderr = io.StringIO()
        with utils.fake_io(data.write):
            sys.stdout.write('hello\n')
            sys.stderr.write('world\n')
            sys.stderr = new_stderr

        out, err = capsys.readouterr()
        assert not out
        assert not err
        assert data.getvalue() == 'hello\nworld\n'

        sys.stdout.write('normal\n')
        sys.stderr.write('still new\n')
        out, err = capsys.readouterr()
        assert out == 'normal\n'
        assert not err
        assert new_stderr.getvalue() == 'still new\n'

    def test_stdout_replaced(self, capsys):
        """Test with replaced stdout."""
        data = io.StringIO()
        new_stdout = io.StringIO()
        with utils.fake_io(data.write):
            sys.stdout.write("hello\n")
            sys.stderr.write("world\n")
            sys.stdout = new_stdout

        out, err = capsys.readouterr()
        assert not out
        assert not err
        assert data.getvalue() == "hello\nworld\n"

        sys.stdout.write("still new\n")
        sys.stderr.write("normal\n")
        out, err = capsys.readouterr()
        assert not out
        assert err == "normal\n"
        assert new_stdout.getvalue() == "still new\n"