def change_env(name, val):
    """
    Args:
        name(str), val(str):

    Returns:
        a context where the environment variable ``name`` being set to
        ``val``. It will be set back after the context exits.
    """
    oldval = os.environ.get(name, None)
    os.environ[name] = val
    yield
    if oldval is None:
        del os.environ[name]
    else:
        os.environ[name] = oldval