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Emit warning on direct instantiation of unmapped MappedAsDataclass by VimalN2005 · Pull Request #13587 · sqlalchemy/sqlalchemy · GitHub

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Emit warning on direct instantiation of unmapped MappedAsDataclass - #13587

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VimalN2005 wants to merge 1 commit into
sqlalchemy:mainfrom
VimalN2005:fix-10064-mapped-as-dataclass-warn
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Emit warning on direct instantiation of unmapped MappedAsDataclass#13587
VimalN2005 wants to merge 1 commit into
sqlalchemy:mainfrom
VimalN2005:fix-10064-mapped-as-dataclass-warn

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VimalN2005 commented Sep 12, 2026

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Fixes

Fixes: #10064

Description

Direct instantiation of unmapped classes (mixins or abstract base classes) using MappedAsDataclass is unsupported as ORM features, column expressions, and relationship attributes cannot be resolved without a concrete mapping.

Per issue discussion and maintainer guidance:

  • Installed an initialization wrapper in _UnmappedDataclassConfig for classes subclassing MappedAsDataclass that emits an SAWarning when the unmapped class is directly instantiated (type(self_) is cls_).
  • Concrete mapped subclasses remain unaffected and instantiate their own dataclass constructors normally without warning.

Checklist

  • Added unit tests in test/orm/declarative/test_dc_transforms.py (UnmappedDataclassInstantiationTest)
  • All 370 tests in test_dc_transforms.py and test_dataclasses.py pass cleanly
  • Added changelog fragment doc/build/changelog/unreleased_21/10064.rst

sqla-tester commented Sep 12, 2026

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Thanks! Issue #10064 is now marked code review in progress and no longer open for pull requests, so this pull request holds the review for it and another one won't land on top of your work. If this pull request is abandoned, a maintainer can put open for pull requests back on #10064 to reopen it to others.

zzzeek requested a review from sqla-tester September 24, 2026 13:04
sqla-tester reviewed Sep 24, 2026
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OK, this is sqla-tester setting up my work on behalf of zzzeek to try to get revision b9e5934 of this pull request into gerrit so we can run tests and reviews and stuff

sqla-tester commented Sep 24, 2026

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New Gerrit review created for change b9e5934: https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/6991

VimalN2005 force-pushed the fix-10064-mapped-as-dataclass-warn branch from b9e5934 to c72f83b Compare September 24, 2026 14:16

VimalN2005 commented Sep 24, 2026

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Hi @zzzeek,

I have addressed the PEP8 (line length / formatting) and PEP484 (typing in decl_base.py) issues flagged by the Gerrit verification pipeline:

  1. PEP8 Compliance: Fixed line lengths in decl_base.py and test_dc_transforms.py, formatted cleanly with black and verified with flake8.
  2. PEP484 Typing: Corrected util.warn() invocation without redundant category argument and used setattr for dynamic __init__ wrapping, resolving all mypy errors.
  3. Unit Tests: All dataclass transform unit tests pass cleanly.

The branch has been updated with commit c72f83b. Ready for Gerrit re-verification. Thank you!

sqla-tester reviewed Sep 24, 2026
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Hi, this is sqla-tester and I see you've pinged me for review. However, user VimalN2005 is not authorized to initiate CI jobs. Please wait for a project member to do this!

zzzeek requested a review from sqla-tester September 24, 2026 17:06
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OK, this is sqla-tester setting up my work on behalf of zzzeek to try to get revision c72f83b of this pull request into gerrit so we can run tests and reviews and stuff

sqla-tester commented Sep 24, 2026

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Patchset c72f83b added to existing Gerrit review https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/6991

sqla-tester reviewed Sep 24, 2026
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Federico Caselli (CaselIT) wrote:

Loiks fine. The sync teat script may require to be run

View this in Gerrit at https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/6991

Comment thread lib/sqlalchemy/orm/decl_base.py
Emit warning on direct instantiation of unmapped MappedAsDataclass
6031fac 
Direct instantiation of unmapped classes (mixins or abstract base
classes) using MappedAsDataclass is unsupported as ORM features and
attributes cannot be resolved without concrete mapping.

An SAWarning is now emitted when instantiating such classes directly.
Concrete mapped subclasses remain unaffected and instantiate normally.

Fixes: sqlalchemy#10064
Change-Id: I5875469671fce1f63e7e93776683e4ba9374c59b
VimalN2005 force-pushed the fix-10064-mapped-as-dataclass-warn branch from c72f83b to 6031fac Compare September 24, 2026 18:51

VimalN2005 commented Sep 24, 2026

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Hi @CaselIT @zzzeek,

I have addressed both review items:

  1. Preload Decorator: Added @util.preload_module("sqlalchemy.orm.decl_api") on _install_unmapped_dataclass_init_warning.
  2. Synchronized Test Suite: Ran tools/sync_test_files.py to synchronize test_dc_transforms_future_anno_sync.py.
  3. Verification: Both normal and future-annotations test suites pass cleanly (6/6 tests), formatted with black and verified with flake8 and mypy.

Updated in commit 6031fac. Ready for review/CI run. Thank you!

sqla-tester reviewed Sep 24, 2026
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Hi, this is sqla-tester and I see you've pinged me for review. However, user VimalN2005 is not authorized to initiate CI jobs. Please wait for a project member to do this!

zzzeek requested a review from sqla-tester September 24, 2026 19:22
sqla-tester reviewed Sep 24, 2026
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OK, this is sqla-tester setting up my work on behalf of zzzeek to try to get revision 6031fac of this pull request into gerrit so we can run tests and reviews and stuff

sqla-tester commented Sep 24, 2026

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Patchset 6031fac added to existing Gerrit review https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/6991

sqla-tester reviewed Sep 24, 2026
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Federico Caselli (CaselIT) wrote:

code review left on gerrit

View this in Gerrit at https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/6991

Comment thread lib/sqlalchemy/orm/decl_base.py
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MappedAsDataclass, prohibit constructor use and/or creation for non-mapped classes, abstract, mixins

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