Joris Van den Bossche created ARROW-17890:
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Summary: [C++][Python] Allow an ExtensionType to register or
implement custom casts
Key: ARROW-17890
URL: https://issues.apache.org/jira/browse/ARROW-17890
Project: Apache Arrow
Issue Type: Improvement
Components: C++, Python
Reporter: Joris Van den Bossche
With ARROW-14500 and ARROW-15545 (https://github.com/apache/arrow/pull/14106),
we are allowing to cast "storage_type" -> "extension<storage_type>" (and the
cast the other way around already worked as well).
Initially, that PR allowed any cast from "any" -> "extension<storage_type>", as
long as the input type could be cast to the storage type (so deferring to the
"any" -> "storage_type" cast). However, because whether a certain cast makes
sense or not depends on the semantics of the extension type, it was restricted
to exactly matching storage_type.
One idea could be to still allow the other casts behind a cast option flag,
like {{allow_non_storage_extension_casts}} (or a better name), so the user can
explicitly allow to cast to/from any type (as long as the cast from/to the
storage type works).
That could help for the user, but for certain casts, the ExtensionType might
also want to control _how_ such a cast is done. For example, for casting
to/from string type (which would be useful for reading/writing CSV files, or
for repr), you typically will want to do something different than casting your
storage array to string.
A more general solution could thus be to have a mechanism for the ExtensionType
to implement a certain cast kernel itself, and register this to the C++ cast
dispatching.
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