Small fix in transpiler initial and final layout

src/qibo/transpiler/pipeline.py

@@ -180,6 +180,8 @@ class Passes:
             Defaults to :math:`qibo.transpiler.unroller.NativeGates.default`.
         on_qubits (list, optional): list of physical qubits to be used.
             If "None" all qubits are used. Defaults to ``None``.
+        int_qubit_name (bool, optional): if `True` the `final_layout` keys are
+            cast to integers.
     """
 
     def __init__(
@@ -188,12 +190,15 @@ def __init__(
         connectivity: nx.Graph = None,
         native_gates: NativeGates = NativeGates.default(),
         on_qubits: list = None,
+        int_qubit_names: bool = False,
     ):
         if on_qubits is not None:
             connectivity = restrict_connectivity_qubits(connectivity, on_qubits)
         self.connectivity = connectivity
         self.native_gates = native_gates
         self.passes = self.default() if passes is None else passes
+        self.initial_layout = None
+        self.int_qubit_names = int_qubit_names
 
     def default(self):
         """Return the default transpiler pipeline for the required hardware connectivity."""
@@ -215,8 +220,12 @@ def default(self):
         return default_passes
 
     def __call__(self, circuit):
-        self.initial_layout = None
-        final_layout = None
+        """
+        This function returns the compiled circuits and the dictionary mapping
+        physical (keys) to logical (values) qubit. If `int_qubit_name` is `True`
+        each key `i` correspond to the `i-th` qubit in the graph.
+        """
+        final_layout = self.initial_layout
         for transpiler_pass in self.passes:
             if isinstance(transpiler_pass, Optimizer):
                 transpiler_pass.connectivity = self.connectivity
@@ -247,7 +256,9 @@ def __call__(self, circuit):
                     TranspilerPipelineError,
                     f"Unrecognised transpiler pass: {transpiler_pass}",
                 )
-
+        # TODO: use directly integers keys
+        if self.int_qubit_names:
+            final_layout = {int(key[1:]): value for key, value in final_layout.items()}
         return circuit, final_layout
 
     def is_satisfied(self, circuit: Circuit):

tests/test_transpiler_pipeline.py

@@ -129,6 +129,16 @@ def test_assert_circuit_equivalence_false():
         assert_circuit_equivalence(circ1, circ2, final_map=final_map)
 
 
+def test_int_qubit_names():
+    circ = Circuit(2)
+    final_map = {i: i for i in range(5)}
+    default_transpiler = Passes(
+        passes=None, connectivity=star_connectivity(), int_qubit_names=True
+    )
+    _, final_layout = default_transpiler(circ)
+    assert final_map == final_layout
+
+
 def test_assert_circuit_equivalence_wrong_nqubits():
     circ1 = Circuit(1)
     circ2 = Circuit(2)