Yifan Mai created BEAM-8519:
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Summary: 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
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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