PR #184442: Fix TD failure under Python 3.9 from runtime PEP 604 union

• Repository: pytorch/pytorch
• Author: huydhn
• State: open | merged: False
• Link: https://github.com/pytorch/pytorch/pull/184442

Description (problem / solution / changelog)

Summary

Fixes #184441.

The target-determination workflow has no actions/setup-python step before the "Do TD" step, so on the self-hosted linux.2xlarge runner python3 resolves to the system Python 3.9 (the boto3 install log in the failing job confirms this: Requirement already satisfied: jmespath ... in /usr/lib/python3.9/site-packages).

tools/testing/target_determination/heuristics/utils.py has from __future__ import annotations, which defers evaluation of annotations — but cast() is a regular function call, so its first argument is evaluated at runtime. PEP 604 union on type objects only works on Python 3.10+; on 3.9 cast(str | None, ...) raises TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'. That exception escapes _get_pr_merge_base and is caught by each heuristic's outer try/except, producing the observed cascade:

Error in EditedByPR: unsupported operand type(s) for |: 'type' and 'NoneType'
Error in HistoricalClassFailurCorrelation: ...
Error in CorrelatedWithHistoricalFailures: ...
Error in HistorialEditedFiles: ...
Error in Profiling: ...
Error in Filepath: ...
Error in PublicBindings: ...

The two offending runtime expressions landed in #182957 (2026-05-08), which is also when TD failures started becoming user-visible (lookup errors are no longer silently turned into empty heuristic results).

Quote the cast targets so the type expression is never evaluated. typing.cast ignores its first argument at runtime, so a forward-reference string works on every Python version.

Test plan

• CI target-determination job runs without the unsupported operand type(s) for | errors.

Authored by Claude.

Changed files

• tools/testing/target_determination/heuristics/utils.py (modified, +2/-2)