取出PCollection中的第一个元素 Args: pcollection (PCollection): 输入PCollection **options: 可配置选项 Returns: PObject: 取出的单个元素，以PObject给出 >>> from bigflow import transforms >>> _p = _pipeline.parallelize([3, 7, 1, 3, 2, 8]) >>> transforms.first(_p).get() 3
(pcollection, **options)
source not stored for this graph (policy: none)