Add more informative message for item assignment error

pytensor/tensor/variable.py

@@ -594,6 +594,11 @@ class _tensor_py_operators:
                     ),
                 )
 
+    def __setitem__(self, key, value):
+        raise TypeError(
+            "TensorVariable does not support item assignment. Use the output of `set` or `add` instead."
+        )
+
     def take(self, indices, axis=None, mode="raise"):
         return at.subtensor.take(self, indices, axis, mode)
 

tests/tensor/test_variable.py

@@ -447,6 +447,15 @@ class TestTensorInstanceMethods:
         with pytest.raises(TypeError, match=msg):
             x.add(y)
 
+    def test_set_item_error(self):
+        x = matrix("x")
+
+        msg = "Use the output of `set` or `add` instead."
+        with pytest.raises(TypeError, match=msg):
+            x[0] = 5
+        with pytest.raises(TypeError, match=msg):
+            x[0] += 5
+
 
 def test_deprecated_import():
     with pytest.warns(