Fix PetabStrPrinter for powers (#421)

Power expressions with non-atomic bases or exponents were not printed correctly. Fixed here.

Author: dweindl

petab/v1/math/printer.py

@@ -37,7 +37,13 @@ def _print_BooleanFalse(self, expr):
     def _print_Pow(self, expr: sp.Pow):
         """Custom printing for the power operator"""
         base, exp = expr.as_base_exp()
-        return f"{self._print(base)} ^ {self._print(exp)}"
+        str_base = self._print(base)
+        str_exp = self._print(exp)
+        if not base.is_Atom:
+            str_base = f"({str_base})"
+        if not exp.is_Atom:
+            str_exp = f"({str_exp})"
+        return f"{str_base} ^ {str_exp}"
 
     def _print_Infinity(self, expr):
         """Custom printing for infinity"""

tests/v1/math/test_math.py

@@ -38,9 +38,11 @@ def test_assumptions():
 
 
 def test_printer():
+    a, b, c, d = sp.symbols("a b c d", real=True)
     assert petab_math_str(None) == ""
     assert petab_math_str(BooleanTrue()) == "true"
     assert petab_math_str(BooleanFalse()) == "false"
+    assert petab_math_str((a + b) ** (c + d)) == "(a + b) ^ (c + d)"
 
 
 def read_cases():