Fix: Solve strange behavior with qubit_properties in Target. (#13400

)

The previous implementation of the Target would always make sure to receive a real value for `num_qubits` or it would default to zero. Mistakenly because of the behavior of `Target::from_configuration` it was assumed that `num_qubits` could be `None` unless otherwise specified. However, the default behavior should be to make `num_qubits` zero unless specified otherwise.

The following commits fix this behavior by fixing the signature of the rust `Target` to have default values and adding an extra check when comparing against `qubit_properties`.

A test casde based on @wshanks findings in #13393 was also included in these additions.

crates/accelerate/src/target_transpiler/mod.rs

@@ -227,18 +227,18 @@ impl Target {
     #[new]
     #[pyo3(signature = (
         description = None,
-        num_qubits = None,
+        num_qubits = 0,
         dt = None,
-        granularity = None,
-        min_length = None,
-        pulse_alignment = None,
-        acquire_alignment = None,
+        granularity = 1,
+        min_length = 1,
+        pulse_alignment = 1,
+        acquire_alignment = 1,
         qubit_properties = None,
         concurrent_measurements = None,
     ))]
     fn new(
         description: Option<String>,
-        num_qubits: Option<usize>,
+        mut num_qubits: Option<usize>,
         dt: Option<f64>,
         granularity: Option<u32>,
         min_length: Option<usize>,
@@ -247,10 +247,9 @@ impl Target {
         qubit_properties: Option<Vec<PyObject>>,
         concurrent_measurements: Option<Vec<Vec<PhysicalQubit>>>,
     ) -> PyResult<Self> {
-        let mut num_qubits = num_qubits;
         if let Some(qubit_properties) = qubit_properties.as_ref() {
-            if let Some(num_qubits) = num_qubits {
-                if num_qubits != qubit_properties.len() {
+            if num_qubits.is_some_and(|num_qubits| num_qubits > 0) {
+                if num_qubits.unwrap() != qubit_properties.len() {
                     return Err(PyValueError::new_err(
                         "The value of num_qubits specified does not match the \
                             length of the input qubit_properties list",

test/python/transpiler/test_target.py

@@ -51,6 +51,7 @@
     Fake7QPulseV1,
 )
 from test import QiskitTestCase  # pylint: disable=wrong-import-order
+from qiskit.providers.backend import QubitProperties
 from test.python.providers.fake_mumbai_v2 import (  # pylint: disable=wrong-import-order
     FakeMumbaiFractionalCX,
 )
@@ -1185,6 +1186,20 @@ def test_target_serialization_preserve_variadic(self):
         # Perform check again, should not throw exception
         self.assertTrue(deserialized_target.instruction_supported("u_var", (0, 1)))
 
+    def test_target_no_num_qubits_qubit_properties(self):
+        """Checks that a Target can be initialized with no qubits but a list of Qubit Properities"""
+
+        # Initialize target qubit properties
+        qubit_properties = [QubitProperties()]
+
+        # Initialize the Target with only a list of qubit properties
+        target = Target(
+            qubit_properties=qubit_properties,
+        )
+
+        # Check that the Target num_qubit attribute matches the length of qubit properties
+        self.assertEqual(target.num_qubits, len(qubit_properties))
+
 
 class TestPulseTarget(QiskitTestCase):
     def setUp(self):