[GitHub] [beam] kennknowles opened a new issue, #18460: TextIO should support reading header lines

[GitBox <gi...@apache.org>]

kennknowles opened a new issue, #18460:
URL: https://github.com/apache/beam/issues/18460

   Users frequently request the ability to skip some header rows when reading text files.
   
   
   https://stackoverflow.com/questions/28450554/skipping-header-rows-is-it-possible-with-cloud-dataflow
   https://stackoverflow.com/questions/43551876/how-do-i-read-and-transform-csv-headers-before-bigqueryio-write
   https://stackoverflow.com/questions/41297704/reading-csv-header-with-dataflow
   https://stackoverflow.com/questions/45554466/google-cloud-dataflow-apache-beam-how-to-process-gzipped-csv-files-with-a-he
   https://stackoverflow.com/questions/44045744/how-do-i-skip-header-files-when-reading-from-google-cloud-storage-in-a-dataflow
   
   This is also relevant for reading file formats such as VCF, see thread https://lists.apache.org/thread.html/dc7e5c3ff20d9270f06c1a298ad949da018a83f900b22d58f6b4c468@%3Cdev.beam.apache.org%3E
   
   Python supports this partially https://github.com/apache/beam/pull/1771/files via skip_header_lines, but the header lines can have useful content, and the number of header lines is not fixed (in VCF).
   
   We should figure out a good API for this and support this natively in TextIO. The API decisions would be:
   
   - How do we specify how much of the beginning of each file is the header: options could be e.g. a certain number of lines; or lines that start with a certain character; or a custom predicate.
   - How do we make the header contents accessible to a user of TextIO. Since the header can be different in each file, we can't return it as a PCollectionView<List<String\>\>. Instead I suppose, when you use a header, you'd need to specify a SerializableFunction<KV<List<String\>, String\>, T\> or something like that for parsing (header, line) -\> user type. Note that currently TextIO.Read does not support returning a user type anyway, so that'd need to be done too.
   
   Imported from Jira [BEAM-2776](https://issues.apache.org/jira/browse/BEAM-2776). Original Jira may contain additional context.
   Reported by: jkff.


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