fix: convert num_features dict keys to Julia Symbols (#1209)

Closes #811

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

Author: MilesCranmerBot

pysr/expression_specs.py

@@ -293,7 +293,7 @@ def julia_expression_options(self):
             num_features = if num_features === nothing
                 nothing
             else
-                (; num_features...)
+                NamedTuple(Symbol(k) => v for (k, v) in num_features)
             end
             structure = SymbolicRegression.TemplateStructure{tuple_symbol}(combine, num_features)
             return (; structure)

pysr/test/test_main.py

@@ -2246,6 +2246,17 @@ def test_process_constraints_swaps_multiplication_constraints(self):
 
 
 class TestTemplateExpressionSpec(unittest.TestCase):
+    def test_num_features_symbol_keys(self):
+        # ponytail: one check — dict keys must reach Julia as Symbols
+        spec = TemplateExpressionSpec(
+            ["f", "g"],
+            "combine(fs, vars) = fs.f(vars[1], vars[2]) + fs.g(vars[3])",
+            {"f": 2, "g": 1},
+        )
+        options = spec.julia_expression_options()
+        names = jl.seval("x -> propertynames(x.structure.num_features)")(options)
+        self.assertEqual(names, (jl.Symbol("f"), jl.Symbol("g")))
+
     def _check_macro_str(self, spec, expected_str):
         self.assertEqual(
             spec._template_macro_str().strip(), dedent(expected_str).strip()