Make LineQubit and GridQubit immutable (#2756)

Coordinates of qubits on line or grid now are properties and can't be changed.

Fixes #2356.

cirq/devices/grid_qubit.py

@@ -38,12 +38,20 @@ class GridQubit(ops.Qid):
     """
 
     def __init__(self, row: int, col: int):
-        self.row = row
-        self.col = col
+        self._row = row
+        self._col = col
 
     def _comparison_key(self):
         return self.row, self.col
 
+    @property
+    def row(self) -> int:
+        return self._row
+
+    @property
+    def col(self) -> int:
+        return self._col
+
     @property
     def dimension(self) -> int:
         return 2

cirq/devices/grid_qubit_test.py

@@ -185,3 +185,13 @@ def test_to_json():
         'row': 5,
         'col': 6,
     }
+
+
+def test_immutable():
+    with pytest.raises(AttributeError, match="can't set attribute"):
+        q = cirq.GridQubit(1, 2)
+        q.col = 3
+
+    with pytest.raises(AttributeError, match="can't set attribute"):
+        q = cirq.GridQubit(1, 2)
+        q.row = 3

cirq/devices/line_qubit.py

@@ -32,11 +32,15 @@ class _BaseLineQid(ops.Qid):
 
     def __init__(self, x: int) -> None:
         """Initializes a line qubit at the given x coordinate."""
-        self.x = x
+        self._x = x
 
     def _comparison_key(self):
         return self.x
 
+    @property
+    def x(self) -> int:
+        return self._x
+
     def with_dimension(self, dimension: int) -> 'LineQid':
         return LineQid(self.x, dimension)
 

cirq/devices/line_qubit_test.py

@@ -258,3 +258,13 @@ def _qid_shape_(self):
         cirq.LineQid(3, 3),
         cirq.LineQid(2, 1),
     ]
+
+
+def test_immutable():
+    with pytest.raises(AttributeError, match="can't set attribute"):
+        q = cirq.LineQubit(5)
+        q.x = 6
+
+    with pytest.raises(AttributeError, match="can't set attribute"):
+        q = cirq.LineQid(5, dimension=4)
+        q.x = 6