fix: Number rich comparisons return NotImplemented for foreign types#19
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Before, `Number("1") == None` (and against lists, objects, etc.) raised
ValueError from the inner C++ expression parser, because str(other) was
fed straight into a Solver. Python's data model expects unknown operand
types to be handled by returning NotImplemented so the other side has a
chance, then by falling back to identity (==) or TypeError (ordering).
Guard __eq__, __lt__, __gt__, __le__, __ge__ with an isinstance check
against (Number, str, int, float). Foreign-type equality falls back to
False; foreign-type ordering raises TypeError as expected.
Regression test in tests/test_number_eq_notimplemented.py covers None,
list, object on the right side, plus existing equal/unequal cases.
See ai/improvements_2026-05-09.md item #5.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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This was referenced May 11, 2026
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Problem.
Number("1") == None(and against lists, objects, etc.) raisedValueErrorfrom the inner C++ expression parser, becausestr(other)was fed straight into aSolver. Python's data model expects unknown operand types to returnNotImplementedso the other side has a chance, then falls back to identity for==orTypeErrorfor ordering.Fix.
src/formula/formula.py— guard__eq__,__lt__,__gt__,__le__,__ge__withisinstance(__value, (Number, str, int, float)). Foreign-type==now falls back toFalse; foreign-type ordering raisesTypeErroras Python expects.Test. New file
tests/test_number_eq_notimplemented.pycoversNone, list, andobject()on the right side, plus the preserved equal/unequal cases forNumber/str/int/float. All pass; full suite 325/325.