Narrow exception catch in _derive_from_expr to stop masking errors

[amc-corey-cox]

Summary

Closes #185

Fixes the broad except Exception in _derive_from_expr that masked real runtime errors as "Expression not in safe subset." Also adds arithmetic coercion for numeric strings and an is_numeric() expression function so curators can handle messy tabular data.

Before

Expression not in safe subset: {phv00112691} / 100.0 * {phv00112686}

After

could not convert string to float: 'A'; class_derivation=Quantity (from pht004041); slot_derivation=value_decimal (from phv00191041)

Changes

Exception handling

• Restricted mode: Only InvalidExpression (simpleeval safety violations) is caught. Real runtime errors (TypeError, ValueError, ZeroDivisionError) propagate as TransformationError with full context (class, slot, populated_from, source row).
• Unrestricted mode: InvalidExpression, TypeError, and ValueError fall through to the asteval fallback (preserving existing behavior for multi-line expressions).

Arithmetic coercion

• Arithmetic operators now coerce numeric strings to float: "100" / "50" → 2.0
• Non-coercible operands return None with a warning: "abc" * 10 → None (logged)
• This is necessary because case() evaluates eagerly — without it, curators can't guard expressions with is_numeric() since the guarded branch would crash before case() checks the condition

New expression function

• is_numeric(x): Returns True if x can be converted to float, False otherwise. Enables guard patterns:

  expr: "case((is_numeric(x), x * 2.54), (True, None))"

Shared helper

• _try_numeric(value): Extracted as a top-level helper for numeric coercion with bool exclusion. Used by both _is_numeric and _null_propagating.

Test plan

• 553 tests pass, 0 failures
• Restricted mode: real errors propagate with actual messages
• Unrestricted mode: asteval fallback works for complex expressions
• Arithmetic coerces numeric strings, returns None for non-numeric with warning
• is_numeric() guard pattern works with case()
• Division by zero still raises (not swallowed)
• Range override tests updated: null → None (null propagation), malformed → raises (restricted mode)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR narrows exception handling in ObjectTransformer._derive_from_expr so restricted evaluation no longer masks genuine runtime/data errors behind a generic “safe subset” message, improving curator-facing diagnostics while preserving the unrestricted asteval fallback behavior.

Changes:

• Update _derive_from_expr to only fall back to asteval when unrestricted_eval=True, while letting real errors propagate in restricted mode to be wrapped as TransformationError with context.
• Remove the misleading “Expression not in safe subset” wrapper behavior in restricted mode.
• Adjust the restricted-mode test to use an expression that is reliably rejected by simpleeval and assert on the real underlying error message.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/linkml_map/transformer/object_transformer.py	Restricts exception catching in expression derivation so restricted mode preserves real error messages and unrestricted mode continues to fall back to asteval.
tests/test_transformer/test_object_transformer.py	Updates restricted-mode test to reflect new error propagation behavior and to use a consistently-rejected expression.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.