对于多个输入PCollection，根据key对PCollection做左连接操作 ，连接结果为(key, (value 1, value 2, ..., value n))，若第m个PCollection没有元素， 则value m为None Args: *pcollections: 输入PCollection **options: 可配置选项 Returns: PCollection: 连接结果 >>> x = _pipeline.parallelize([("a", 1), ("b", 4)
(*pcollections, **options)
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