Path.rename() is simple wrapper around os.rename() and returns the result of os.rename(). If os.rename() became returning something useful, this change will made impossible to return it from Path.rename().

Why not create destination Path object explicitly?

    bar_var = pathlib.PosixPath('bar')
    foo_var = pathlib.PosixPath('foo')
    bar_var.rename(foo_var)

Note, that in 3.6 you can also write

    os.rename(bar_var, foo_var)