def chdir(new_dir):
    """
    stolen from bcbio.
    Context manager to temporarily change to a new directory.

    http://lucentbeing.com/blog/context-managers-and-the-with-statement-in-python/
    """
    cur_dir = os.getcwd()
    _mkdir(new_dir)
    os.chdir(new_dir)
    try:
        yield
    finally:
        os.chdir(cur_dir)