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gh-131933: Document UnionType/Union merger in What's New #131941

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Merged
JelleZijlstra merged 6 commits into from
Apr 4, 2025
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gh-131933: Document UnionType/Union merger in What's New #131941

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gh-131933: Document UnionType/Union merger in What's New
JelleZijlstra Mar 31, 2025
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Update Doc/whatsnew/3.14.rst
JelleZijlstra Mar 31, 2025
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Update Doc/whatsnew/3.14.rst
JelleZijlstra Mar 31, 2025
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Merge remote-tracking branch 'upstream/main' into union-docs
JelleZijlstra Apr 4, 2025
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@JelleZijlstra
JelleZijlstra committed Apr 4, 2025
commit 151223da03ecaa4536359499793b7a011b2a16fb
11 changes: 7 additions & 4 deletions Doc/whatsnew/3.14.rst
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Expand Up @@ -1103,10 +1103,13 @@ typing
return two different objects. Users should use ``==`` to compare unions for equality, not
``is``. New-style unions have never been cached this way.
This change could increase memory usage for some programs that use a large number of
unions, but unions used in annotations are no longer evaluated by default in Python 3.14
(because of :pep:`649`), the new union object is itself much smaller, and removing the
cache also saves some space, so it is unlikely that this change will cause a significant
increase in memory usage.
unions created by subscripting ``typing.Union``. However, several factors offset this cost:
unions used in annotations are no longer evaluated by default in Python 3.14
because of :pep:`649`; an instance of :class:`types.UnionType` is
itself much smaller than the object returned by ``Union[]`` was on prior Python
versions; and removing the cache also saves some space. It is therefore
unlikely that this change will cause a significant increase in memory usage for most
users.
- Previously, old-style unions were implemented using the private class
``typing._UnionGenericAlias``. This class is no longer needed for the implementation,
but it has been retained for backward compatibility, with removal scheduled for Python
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