ENH: set data keys as first positional arguments (#488)

src/tensorwaves/function/sympy/__init__.py

@@ -109,7 +109,10 @@ def create_parametrized_function(
       [0.0, 0.0, 0.0, 0.0, 0.0]
     """
     free_symbols = _get_free_symbols(expression)
-    sorted_symbols = sorted(free_symbols, key=lambda s: s.name)
+    parameter_set = set(parameters)
+    parameter_symbols = sorted(free_symbols & parameter_set, key=lambda s: s.name)
+    data_symbols = sorted(free_symbols - parameter_set, key=lambda s: s.name)
+    sorted_symbols = tuple(data_symbols + parameter_symbols)  # for partial+gradient
     lambdified_function = _lambdify_normal_or_fast(
         expression=expression,
         symbols=sorted_symbols,

tests/function/test_function.py

@@ -34,7 +34,8 @@ def function(self) -> ParametrizedBackendFunction:
         return create_parametrized_function(expression, parameters, backend="numpy")
 
     def test_argument_order(self, function: ParametrizedBackendFunction):
-        assert function.argument_order == ("c_1", "c_2", "c_3", "c_4", "x")
+        """Test whether data arguments come before parameters."""
+        assert function.argument_order == ("x", "c_1", "c_2", "c_3", "c_4")
 
     @pytest.mark.parametrize(
         ("test_data", "expected_results"),

tests/test_estimator.py

@@ -112,7 +112,7 @@ def test_create_cached_function(backend):
 
     assert isinstance(cached_function, ParametrizedBackendFunction)
     assert isinstance(cache_transformer, SympyDataTransformer)
-    assert cached_function.argument_order == ("a", "c", "f0", "x")
+    assert cached_function.argument_order == ("f0", "x", "a", "c")  # data args first
     assert set(cached_function.parameters) == {"a", "c"}
     assert set(cache_transformer.functions) == {"f0", "x"}