add exceptions for common mistakes, and a test

circkit/circuit.py

@@ -328,6 +328,16 @@ def clone_empty(self, base_ring=None, name=None):
     # --------------------------------
 
     def add_node(self, node):
+        if not isinstance(node, Node):
+            if isinstance(node, Operation):
+                raise TypeError(
+                    "Trying to add Operation as node, need to create a node from it."
+                    " Call the operation with its node arguments."
+                )
+            raise TypeError(
+                f"Trying to add {node:r} as node, but it is not a Node."
+            )
+
         if node.circuit is not self:
             raise RuntimeError(f"node's circuit {node.circuit} is not self")
 
@@ -390,6 +400,10 @@ def add_output(self, value):
             raise TypeError(
                 f"Can not output the Node {value} from a different circuit"
             )
+        elif isinstance(value, Operation):
+            raise TypeError(
+                "Can not output Operation, need to create a node. Call the operation with its node arguments."
+            )
 
         try:
             nodes = list(value)

tests/test_types.py

@@ -0,0 +1,26 @@
+import random
+import pytest
+
+from circkit.arithmetic import ArithmeticCircuit
+
+def test_random():
+    C = ArithmeticCircuit(name="TwoRandoms")
+
+    # note: add_node is not needed to be called usually
+    op5 = C.CONST(5)
+    with pytest.raises(TypeError):
+        C.add_node(op5)
+    with pytest.raises(TypeError):
+        C.add_output(op5)
+
+    # correct usage (create node)
+    C.add_output(op5())
+
+    op_rand = C.RND()
+    with pytest.raises(TypeError):
+        C.add_node(op_rand)
+    with pytest.raises(TypeError):
+        C.add_output(op_rand)
+
+    # correct usage (create node)
+    C.add_output(op_rand())