Python: Fix PowerFx eval crash on non-English system locales by setting CurrentUICulture to en-US

[moonbox3]

Motivation and Context

On non-English system locales, PowerFx emits localized error messages for undefined variables (e.g., Italian instead of English). The existing error-handling guards only match English strings like "isn't recognized" and "Name isn't valid", so localized messages fall through and crash the workflow.

Fixes #4321

Description

The root cause is that CurrentUICulture controls the language of .NET error messages, but only CurrentCulture was being set to en-US before calling engine.eval. This fix saves and overrides CurrentUICulture to en-US alongside CurrentCulture, ensuring PowerFx error messages are always in English and match the existing string guards. Both cultures are restored in the finally block. A regression test simulates a non-English UI culture (it-IT) and verifies that undefined variable references return None instead of raising.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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Note: PR autogenerated by moonbox3's agent

[moonbox3]

Automated Code Review

Reviewers: 3 | Confidence: 89%

✓ Correctness

The diff correctly saves and restores CurrentUICulture alongside CurrentCulture before calling engine.eval(), ensuring PowerFx error messages are always in English so the downstream string-matching error handlers work on non-English systems. The finally block properly restores both culture values even when exceptions propagate to the outer except ValueError. The regression test is well-structured and mirrors the existing test patterns. No correctness issues found.

✓ Security Reliability

The diff correctly extends the existing culture save/restore pattern to also cover CurrentUICulture, preventing localized PowerFx error messages from bypassing English string guards. The finally block properly restores both culture values on all exit paths. No new security or reliability issues are introduced; the pre-existing thread-safety concern around mutating global/thread-local culture state is not worsened by this change.

✓ Test Coverage

The diff adds CurrentUICulture handling alongside CurrentCulture in the PowerFx eval path and includes a well-structured regression test that sets a non-English UI culture and verifies undefined variables still return None. The test directly exercises the bug scenario. However, there is no assertion verifying that CurrentUICulture is properly restored after eval() returns, which is the other half of the production-code change (the finally block). Without that, a regression in the cleanup path would go undetected.

Suggestions

• The mutation of CultureInfo.CurrentCulture and CultureInfo.CurrentUICulture is not thread-safe if multiple threads call eval concurrently. This is a pre-existing concern not introduced by this diff, but worth noting for future hardening (e.g., using a lock around the eval call).
• Add an assertion after state.eval() to verify that CurrentUICulture is restored to the pre-eval value (e.g., 'it-IT'). This would cover the finally-block restoration logic in the production code and catch future regressions in cleanup.
• Consider adding a parallel test (or extending this one) that exercises a successful eval (not just the undefined-variable error path) under a non-English UI culture, to confirm the culture-swap works for normal evaluation too.

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Automated review by moonbox3's agents

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/declarative/agent_framework_declarative/_workflows
_declarative_base.py	389	38	90%	44, 47, 134, 212, 226, 240, 251, 268, 407, 435, 518–520, 522–524, 526, 529, 536, 562, 586–590, 592–600, 621–622, 624–625
TOTAL	22226	2739	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
4718	247 💤	0 ❌	0 🔥	1m 19s ⏱️

[Copilot]

Pull request overview

Fixes a locale-dependent crash in the declarative PowerFx evaluator by ensuring .NET UI culture is forced to English during evaluation, so existing (English) error-string guards behave consistently across non-English systems.

Changes:

• In DeclarativeWorkflowState.eval(), temporarily sets CultureInfo.CurrentUICulture to en-US alongside CurrentCulture and restores both afterward.
• Adds a regression test that simulates a non-English UI culture (it-IT) and verifies undefined variables return None instead of raising.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py	Forces CurrentUICulture to en-US during PowerFx engine.eval() to prevent localized error messages from bypassing the undefined-variable guard.
python/packages/declarative/tests/test_powerfx_yaml_compatibility.py	Adds a regression test for non-English UI cultures to ensure undefined variable references are handled gracefully.