利用给定的key_extractor和value_extractor对输入PCollection分组，返回一个表示分组结果的PTable Args: pcollection (PCollection): 输入PCollection key_extractor (function): 用于提取key的函数 value_extractor (function, optional): 用于提取value的函数 **options: 可配置选项 Returns: PTable: 分组结果 >>
(pcollection, key_extractor, value_extractor=None, **options)
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