def apple_deployment_target_env(os_, os_version):
    """environment variable name which controls deployment target"""
    env_name = {'Macos': 'MACOSX_DEPLOYMENT_TARGET', 'iOS':
        'IOS_DEPLOYMENT_TARGET', 'watchOS': 'WATCHOS_DEPLOYMENT_TARGET',
        'tvOS': 'TVOS_DEPLOYMENT_TARGET'}.get(str(os_))
    if not env_name:
        return {}
    return {env_name: os_version}
