diff --git a/hugr-py/src/hugr/cfg.py b/hugr-py/src/hugr/cfg.py
index 7c46c9abd..9ee06d49a 100644
--- a/hugr-py/src/hugr/cfg.py
+++ b/hugr-py/src/hugr/cfg.py
@@ -10,7 +10,7 @@
 
 from hugr import ops, tys, val
 
-from .dfg import _DfBase
+from .dfg import DfBase
 from .exceptions import MismatchedExit, NoSiblingAncestor, NotInSameCfg
 from .hugr import Hugr, ParentBuilder
 
@@ -19,7 +19,7 @@
     from .tys import Type, TypeRow
 
 
-class Block(_DfBase[ops.DataflowBlock]):
+class Block(DfBase[ops.DataflowBlock]):
     """Builder class for a basic block in a HUGR control flow graph."""
 
     def set_outputs(self, *outputs: Wire) -> None:
diff --git a/hugr-py/src/hugr/cond_loop.py b/hugr-py/src/hugr/cond_loop.py
index 9548f2a10..ca286fcc4 100644
--- a/hugr-py/src/hugr/cond_loop.py
+++ b/hugr-py/src/hugr/cond_loop.py
@@ -13,7 +13,7 @@
 from hugr import ops
 from hugr.tys import Sum
 
-from .dfg import _DfBase
+from .dfg import DfBase
 from .hugr import Hugr, ParentBuilder
 
 if TYPE_CHECKING:
@@ -21,7 +21,7 @@
     from .tys import TypeRow
 
 
-class Case(_DfBase[ops.Case]):
+class Case(DfBase[ops.Case]):
     """Dataflow graph builder for a case in a conditional."""
 
     _parent_cond: Conditional | None = None
@@ -202,7 +202,7 @@ def add_case(self, case_id: int) -> Case:
 
 
 @dataclass
-class TailLoop(_DfBase[ops.TailLoop]):
+class TailLoop(DfBase[ops.TailLoop]):
     """Builder for a tail-controlled loop.
 
     Args:
diff --git a/hugr-py/src/hugr/dfg.py b/hugr-py/src/hugr/dfg.py
index 76ca42b2a..a2b4dbb0e 100644
--- a/hugr-py/src/hugr/dfg.py
+++ b/hugr-py/src/hugr/dfg.py
@@ -30,7 +30,7 @@
 
 
 @dataclass()
-class _DefinitionBuilder(Generic[OpVar]):
+class DefinitionBuilder(Generic[OpVar]):
     """Base class for builders that can define functions, constants, and aliases.
 
     As this class may be a root node, it does not extend `ParentBuilder`.
@@ -95,7 +95,7 @@ def add_alias_defn(self, name: str, ty: Type, parent: ToNode | None = None) -> N
 
 
 @dataclass()
-class _DfBase(ParentBuilder[DP], _DefinitionBuilder, AbstractContextManager):
+class DfBase(ParentBuilder[DP], DefinitionBuilder, AbstractContextManager):
     """Base class for dataflow graph builders.
 
     Args:
@@ -636,7 +636,7 @@ def _wire_up_port(self, node: Node, offset: PortOffset, p: Wire) -> tys.Type:
         return self._get_dataflow_type(src)
 
 
-class Dfg(_DfBase[ops.DFG]):
+class Dfg(DfBase[ops.DFG]):
     """Builder for a simple nested Dataflow graph, with root node of type
     :class:`DFG <hugr.ops.DFG>`.
 
@@ -672,7 +672,7 @@ def _ancestral_sibling(h: Hugr, src: Node, tgt: Node) -> Node | None:
 
 
 @dataclass
-class Function(_DfBase[ops.FuncDefn]):
+class Function(DfBase[ops.FuncDefn]):
     """Build a function definition as a HUGR dataflow graph.
 
     Args:
diff --git a/hugr-py/src/hugr/function.py b/hugr-py/src/hugr/function.py
index e95042655..8e31b50b8 100644
--- a/hugr-py/src/hugr/function.py
+++ b/hugr-py/src/hugr/function.py
@@ -6,7 +6,7 @@
 from typing import TYPE_CHECKING
 
 from . import ops
-from .dfg import Function, _DefinitionBuilder
+from .dfg import DefinitionBuilder, Function
 from .hugr import Hugr
 
 if TYPE_CHECKING:
@@ -17,7 +17,7 @@
 
 
 @dataclass
-class Module(_DefinitionBuilder[ops.Module]):
+class Module(DefinitionBuilder[ops.Module]):
     """Build a top-level HUGR module.
 
     Examples:
diff --git a/hugr-py/src/hugr/hugr.py b/hugr-py/src/hugr/hugr.py
index 2873cb258..7ebf9a6a8 100644
--- a/hugr-py/src/hugr/hugr.py
+++ b/hugr-py/src/hugr/hugr.py
@@ -64,7 +64,7 @@ def to_serial(self, node: Node) -> SerialOp:
 P = TypeVar("P", InPort, OutPort)
 K = TypeVar("K", InPort, OutPort)
 OpVar = TypeVar("OpVar", bound=Op)
-OpVar2 = TypeVar("OpVar2", bound=Op)
+OpVarCov = TypeVar("OpVarCov", bound=Op, covariant=True)
 
 
 class ParentBuilder(ToNode, Protocol[OpVar]):
@@ -85,7 +85,7 @@ def parent_op(self) -> OpVar:
 
 
 @dataclass()
-class Hugr(Mapping[Node, NodeData], Generic[OpVar]):
+class Hugr(Mapping[Node, NodeData], Generic[OpVarCov]):
     """The core HUGR datastructure.
 
     Args:
@@ -108,7 +108,7 @@ class Hugr(Mapping[Node, NodeData], Generic[OpVar]):
     # List of free node indices, populated when nodes are deleted.
     _free_nodes: list[Node]
 
-    def __init__(self, root_op: OpVar | None = None) -> None:
+    def __init__(self, root_op: OpVarCov | None = None) -> None:
         self._free_nodes = []
         self._links = BiMap()
         self._nodes = []
@@ -134,7 +134,7 @@ def __iter__(self) -> Iterator[Node]:
     def __len__(self) -> int:
         return self.num_nodes()
 
-    def _get_typed_op(self, node: ToNode, cl: type[OpVar2]) -> OpVar2:
+    def _get_typed_op(self, node: ToNode, cl: type[OpVar]) -> OpVar:
         op = self[node].op
         assert isinstance(op, cl)
         return op
@@ -329,7 +329,7 @@ def delete_link(self, src: OutPort, dst: InPort) -> None:
             return
         # TODO make sure sub-offset is handled correctly
 
-    def root_op(self) -> OpVar:
+    def root_op(self) -> OpVarCov:
         """The operation of the root node.
 
         Examples:
@@ -337,7 +337,7 @@ def root_op(self) -> OpVar:
             >>> h.root_op()
             Module()
         """
-        return cast(OpVar, self[self.root].op)
+        return cast(OpVarCov, self[self.root].op)
 
     def num_nodes(self) -> int:
         """The number of nodes in the HUGR.