Fix PetabStrPrinter for non-integer rational exponents#489
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dweindl merged 2 commits intoJun 22, 2026
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A non-integer Rational exponent (e.g. a square root, exponent 1/2) is a sympy Atom but prints as the multi-token "1/2", so PetabStrPrinter emitted `sqrt(a)` as the unparenthesized `a ^ 1/2`. Since `^` binds tighter than `/`, that re-parses as `(a^1)/2 = a/2` -- a silent round-trip corruption (`petab_math_str(sympify_petab(...))` is not the identity for square roots). The `not exp.is_Atom` guard added in PEtab-dev#421 covers non-atomic exponents but not this atomic-yet-multi-token case; parenthesize a non-integer rational exponent explicitly, so `petab_math_str(sqrt(a)) == "a ^ (1/2)"`, which re-parses correctly. Integer powers and the PEtab-dev#421 cases are unchanged.
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petab_math_str(sympify_petab(x))is not the identity for square roots and other non-integer rational exponents. For example,petab_math_str(sqrt(a))returnsa ^ 1/2, whichsympify_petabre-parses as(a^1)/2 = a/2. This is a silent mangling of the expression.The cause is that a non-integer
Rationalexponent, such as1/2, is a sympyAtombut prints as the multi-token string1/2, so thenot exp.is_Atomguard added in #421 does not parenthesize it. Since^binds tighter than/, the unparenthesizeda ^ 1/2then parses as(a^1)/2.This fix parenthesizes a non-integer rational exponent explicitly, so
petab_math_str(sqrt(a)) == "a ^ (1/2)", which re-parses correctly. Integer powers and the non-atomic base/exponent cases from #421 are unchanged.