Fix PetabStrPrinter for non-integer rational exponents (#489)

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 #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 #421 cases are unchanged.

Author: wshlavacek

petab/v1/math/printer.py

@@ -41,7 +41,10 @@ def _print_Pow(self, expr: sp.Pow):
         str_exp = self._print(exp)
         if not base.is_Atom:
             str_base = f"({str_base})"
-        if not exp.is_Atom:
+        # A non-integer Rational exponent (e.g. sqrt -> 1/2) is an Atom but
+        # prints as the multi-token "1/2", so without parentheses "x ^ 1/2"
+        # re-parses as (x^1)/2. Parenthesize it explicitly.
+        if not exp.is_Atom or (exp.is_Rational and not exp.is_Integer):
             str_exp = f"({str_exp})"
         return f"{str_base} ^ {str_exp}"
 

tests/v1/math/test_math.py

@@ -43,6 +43,10 @@ def test_printer():
     assert petab_math_str(BooleanTrue()) == "true"
     assert petab_math_str(BooleanFalse()) == "false"
     assert petab_math_str((a + b) ** (c + d)) == "(a + b) ^ (c + d)"
+    # A non-integer rational exponent must be parenthesized, else "a ^ 1/2"
+    # re-parses as (a^1)/2 (i.e. sqrt(a) would round-trip to a/2).
+    assert petab_math_str(sp.sqrt(a)) == "a ^ (1/2)"
+    assert petab_math_str(a ** sp.Rational(2, 3)) == "a ^ (2/3)"
 
 
 def read_cases():