Ensure that variable names are not sympy functions

pysr/sr.py

@@ -169,6 +169,12 @@ def _check_assertions(
         assert X.shape[0] == weights.shape[0]
     if use_custom_variable_names:
         assert len(variable_names) == X.shape[1]
+        # Check none of the variable names are function names:
+        for var_name in variable_names:
+            if var_name in sympy_mappings or var_name in sympy.__dict__.keys():
+                raise ValueError(
+                    f"Variable name {var_name} is already a function name."
+                )
 
 
 def best(*args, **kwargs):  # pragma: no cover

test/test.py

@@ -546,6 +546,13 @@ class TestMiscellaneous(unittest.TestCase):
         with self.assertRaises(ValueError):
             model.fit(X, y)
 
+    def test_sympy_function_fails_as_variable(self):
+        model = PySRRegressor()
+        X = np.random.randn(100, 2)
+        y = np.random.randn(100)
+        with self.assertRaises(ValueError):
+            model.fit(X, y, variable_names=["x1", "N"])
+
     def test_pickle_with_temp_equation_file(self):
         """If we have a temporary equation file, unpickle the estimator."""
         model = PySRRegressor(