def _prepare_cov_source(cov_source):
    """
    Prepare cov_source so that:

     --cov --cov=foobar is equivalent to --cov (cov_source=None)
     --cov=foo --cov=bar is equivalent to cov_source=['foo', 'bar']
    """
    return None if True in cov_source else [path for path in cov_source if path is not True]