[PorList] Error if multiple ports are renamed to the same name

masque/ports.py

@@ -331,6 +331,10 @@ class PortList(metaclass=ABCMeta):
         missing = set(mapping) - set(self.ports)
         if missing:
             raise PortError(f'Ports to rename were not found: {missing}')
+        renamed_targets = [vv for vv in mapping.values() if vv is not None]
+        duplicate_targets = {vv for vv in renamed_targets if renamed_targets.count(vv) > 1}
+        if duplicate_targets:
+            raise PortError(f'Renamed ports would collide: {duplicate_targets}')
 
         for kk, vv in mapping.items():
             if vv is None or vv != kk:

masque/test/test_ports.py

@@ -89,6 +89,25 @@ def test_port_list_rename_missing_port_raises() -> None:
     assert set(pl.ports) == {"A"}
 
 
+def test_port_list_rename_colliding_targets_raises() -> None:
+    class MyPorts(PortList):
+        def __init__(self) -> None:
+            self._ports = {"A": Port((0, 0), 0), "B": Port((1, 0), 0)}
+
+        @property
+        def ports(self) -> dict[str, Port]:
+            return self._ports
+
+        @ports.setter
+        def ports(self, val: dict[str, Port]) -> None:
+            self._ports = val
+
+    pl = MyPorts()
+    with pytest.raises(PortError, match="Renamed ports would collide"):
+        pl.rename_ports({"A": "C", "B": "C"})
+    assert set(pl.ports) == {"A", "B"}
+
+
 def test_port_list_add_port_pair_requires_distinct_names() -> None:
     class MyPorts(PortList):
         def __init__(self) -> None: