def get_default_sources(config):
    """
        Returns the default sources in a config. This is all sources that
        do not have `built_by_default` set to false.
    """

    sources = []

    for source, src_config in config['sources'].items():
        if src_config.get('options', {}).get('build_by_default', True):
            sources.append(source)

    return sources