从一个SchemaPCollection转化成一个每个元素是tuple的PCollection Args: pcollection (SchemaPCollection): 输入一个SchemaPCollection(FieldsDict) fields (Iterable): 一个列表、元组、一个逗号分隔的字符串,表示要输出的字段 Returns: PCollection: 每个元素是fields对应的值构成的tuple
(pcollection, fields)
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