diff --git a/hugr-py/src/hugr/_dfg.py b/hugr-py/src/hugr/_dfg.py
index f0658f397..54fd20888 100644
--- a/hugr-py/src/hugr/_dfg.py
+++ b/hugr-py/src/hugr/_dfg.py
@@ -108,7 +108,7 @@ def insert_cfg(self, cfg: Cfg, *args: Wire) -> Node:
 
     def set_outputs(self, *args: Wire) -> None:
         self._wire_up(self.output_node, args)
-        self.parent_op()._set_out_types(self._output_op().types)
+        self.parent_op()._set_out_types(self._output_op()._types())
 
     def add_state_order(self, src: Node, dst: Node) -> None:
         # adds edge to the right of all existing edges
@@ -116,8 +116,8 @@ def add_state_order(self, src: Node, dst: Node) -> None:
 
     def _wire_up(self, node: Node, ports: Iterable[Wire]):
         tys = [self._wire_up_port(node, i, p) for i, p in enumerate(ports)]
-        if isinstance(op := self.hugr[node].op, ops.DataflowOp):
-            op._set_in_types(tys)
+        if isinstance(op := self.hugr[node].op, ops.PartialOp):
+            op.set_in_types(tys)
 
     def _get_dataflow_type(self, wire: Wire) -> Type:
         port = wire.out_port()
diff --git a/hugr-py/src/hugr/_exceptions.py b/hugr-py/src/hugr/_exceptions.py
index e5c211d44..f003fc6c7 100644
--- a/hugr-py/src/hugr/_exceptions.py
+++ b/hugr-py/src/hugr/_exceptions.py
@@ -34,3 +34,8 @@ def msg(self):
 
 class ParentBeforeChild(Exception):
     msg: str = "Parent node must be added before child node."
+
+
+@dataclass
+class IncompleteOp(Exception):
+    msg: str = "Operation is incomplete, may require set_in_types to be called."
diff --git a/hugr-py/src/hugr/_ops.py b/hugr-py/src/hugr/_ops.py
index 149db8680..268394223 100644
--- a/hugr-py/src/hugr/_ops.py
+++ b/hugr-py/src/hugr/_ops.py
@@ -6,6 +6,7 @@
 import hugr.serialization.ops as sops
 from hugr.utils import ser_it
 import hugr._tys as tys
+from ._exceptions import IncompleteOp
 
 if TYPE_CHECKING:
     from hugr._hugr import Hugr, Node, Wire, _Port
@@ -39,13 +40,15 @@ def port_type(self, port: _Port) -> tys.Type:
             return sig.input[port.offset]
         return sig.output[port.offset]
 
-    def _set_in_types(self, types: tys.TypeRow) -> None:
-        return
-
     def __call__(self, *args) -> Command:
         return Command(self, list(args))
 
 
+@runtime_checkable
+class PartialOp(DataflowOp, Protocol):
+    def set_in_types(self, types: tys.TypeRow) -> None: ...
+
+
 @dataclass(frozen=True)
 class Command:
     op: DataflowOp
@@ -88,16 +91,21 @@ def __call__(self) -> Command:
 
 
 @dataclass()
-class Output(DataflowOp):
-    types: tys.TypeRow = field(default_factory=list)
+class Output(PartialOp):
+    types: tys.TypeRow | None = None
+
+    def _types(self) -> tys.TypeRow:
+        if self.types is None:
+            raise IncompleteOp()
+        return self.types
 
     def to_serial(self, node: Node, parent: Node, hugr: Hugr) -> sops.Output:
-        return sops.Output(parent=parent.idx, types=ser_it(self.types))
+        return sops.Output(parent=parent.idx, types=ser_it(self._types()))
 
     def outer_signature(self) -> tys.FunctionType:
-        return tys.FunctionType(input=self.types, output=[])
+        return tys.FunctionType(input=self._types(), output=[])
 
-    def _set_in_types(self, types: tys.TypeRow) -> None:
+    def set_in_types(self, types: tys.TypeRow) -> None:
         self.types = types
 
 
@@ -128,23 +136,28 @@ def outer_signature(self) -> tys.FunctionType:
 
 
 @dataclass()
-class MakeTupleDef(DataflowOp):
-    types: tys.TypeRow = field(default_factory=list)
+class MakeTupleDef(PartialOp):
+    types: tys.TypeRow | None = None
     num_out: int | None = 1
 
+    def _types(self) -> tys.TypeRow:
+        if self.types is None:
+            raise IncompleteOp()
+        return self.types
+
     def to_serial(self, node: Node, parent: Node, hugr: Hugr) -> sops.MakeTuple:
         return sops.MakeTuple(
             parent=parent.idx,
-            tys=ser_it(self.types),
+            tys=ser_it(self._types()),
         )
 
     def __call__(self, *elements: Wire) -> Command:
         return super().__call__(*elements)
 
     def outer_signature(self) -> tys.FunctionType:
-        return tys.FunctionType(input=self.types, output=[tys.Tuple(*self.types)])
+        return tys.FunctionType(input=self._types(), output=[tys.Tuple(*self._types())])
 
-    def _set_in_types(self, types: tys.TypeRow) -> None:
+    def set_in_types(self, types: tys.TypeRow) -> None:
         self.types = types
 
 
@@ -152,9 +165,14 @@ def _set_in_types(self, types: tys.TypeRow) -> None:
 
 
 @dataclass()
-class UnpackTupleDef(DataflowOp):
+class UnpackTupleDef(PartialOp):
     types: tys.TypeRow = field(default_factory=list)
 
+    def _types(self) -> tys.TypeRow:
+        if self.types is None:
+            raise IncompleteOp()
+        return self.types
+
     @property
     def num_out(self) -> int | None:
         return len(self.types)
@@ -171,7 +189,7 @@ def __call__(self, tuple_: Wire) -> Command:
     def outer_signature(self) -> tys.FunctionType:
         return MakeTupleDef(self.types).outer_signature().flip()
 
-    def _set_in_types(self, types: tys.TypeRow) -> None:
+    def set_in_types(self, types: tys.TypeRow) -> None:
         (t,) = types
         assert isinstance(t, tys.Sum), f"Expected unary Sum, got {t}"
         (row,) = t.variant_rows
@@ -203,8 +221,7 @@ def outer_signature(self) -> tys.FunctionType:
 class DfParentOp(Op, Protocol):
     def inner_signature(self) -> tys.FunctionType: ...
 
-    def _set_out_types(self, types: tys.TypeRow) -> None:
-        return
+    def _set_out_types(self, types: tys.TypeRow) -> None: ...
 
 
 @dataclass()