Remove list searches in CouplingMap.distance()

qiskit/transpiler/coupling.py

@@ -50,6 +50,8 @@ def __init__(self, couplinglist=None, description=None):
         self._dist_matrix = None
         # a sorted list of physical qubits (integers) in this coupling map
         self._qubit_list = None
+        # number of qubits in the graph
+        self._size = None
         # a sorted list of physical qubits (integers) in this coupling map
         self._is_symmetric = None
 
@@ -58,7 +60,9 @@ def __init__(self, couplinglist=None, description=None):
 
     def size(self):
         """Return the number of physical qubits in this graph."""
-        return len(self.graph)
+        if self._size is None:
+            self._size = len(self.graph)
+        return self._size
 
     def get_edges(self):
         """
@@ -85,6 +89,7 @@ def add_physical_qubit(self, physical_qubit):
         self.graph.add_node(physical_qubit)
         self._dist_matrix = None  # invalidate
         self._qubit_list = None  # invalidate
+        self._size = None  # invalidate
 
     def add_edge(self, src, dst):
         """
@@ -169,10 +174,10 @@ def distance(self, physical_qubit1, physical_qubit2):
         Raises:
             CouplingError: if the qubits do not exist in the CouplingMap
         """
-        if physical_qubit1 not in self.physical_qubits:
-            raise CouplingError("%s not in coupling graph" % (physical_qubit1,))
-        if physical_qubit2 not in self.physical_qubits:
-            raise CouplingError("%s not in coupling graph" % (physical_qubit2,))
+        if physical_qubit1 >= self.size():
+            raise CouplingError("%s not in coupling graph" % physical_qubit1)
+        if physical_qubit2 >= self.size():
+            raise CouplingError("%s not in coupling graph" % physical_qubit2)
         if self._dist_matrix is None:
             self._compute_distance_matrix()
         return int(self._dist_matrix[physical_qubit1, physical_qubit2])