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https://issues.apache.org/jira/browse/BEAM-8519?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17548271#comment-17548271
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Danny McCormick commented on BEAM-8519:
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This issue has been migrated to https://github.com/apache/beam/issues/19913
> Typehint for Python CombineFn Accumulator
> -----------------------------------------
>
> Key: BEAM-8519
> URL: https://issues.apache.org/jira/browse/BEAM-8519
> Project: Beam
> Issue Type: New Feature
> Components: sdk-py-core
> Reporter: Yifan Mai
> Priority: P3
> Labels: types
>
> Feature request:
> Allow users to specify a typehint for the accumulator type of CombineFn,
> either through a typehints decorator, or by using Python type annotations on
> the CombineFn methods, or through inheriting a CombineFn [generic
> type|https://docs.python.org/3/library/typing.html#user-defined-generic-types].
> Benefits:
> * Allow the user to [specify a more efficient
> coder|https://beam.apache.org/documentation/programming-guide/#data-encoding-and-type-safety]
> for accumulators if one is available. Currently users are able to do this
> for the input and output elements, but not the accumulators, which [always
> uses the fallback
> coder|https://github.com/apache/beam/blob/d664592ee761d45b0273edb027a18c6ed9340349/sdks/python/apache_beam/transforms/core.py#L859-L860].
> * Allows typechecking of methods for CombineFn e.g. check that
> create_accumulator(), add_input() and merge_accumulators() returns
> accumulators. This provides better developer ergonomics as the CombineFnAPI
> is non-trivial.
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