Fix: Comments from Matthew's review

- Mention duplication in docstring for rust Target.
- Use f"{*:g}" to avoid printing the floating point for 0 in `Target`'s repr method.
- Add note mentioning future unit-tests in rust.

crates/accelerate/src/target_transpiler/mod.rs

@@ -32,6 +32,7 @@ use pyo3::{
 };
 
 use qiskit_circuit::circuit_instruction::convert_py_to_operation_type;
+
 use qiskit_circuit::operations::{Operation, OperationType, Param};
 use smallvec::SmallVec;
 
@@ -54,6 +55,8 @@ type GateMap = IndexMap<String, PropsMap, RandomState>;
 type PropsMap = NullableIndexMap<Qargs, Option<InstructionProperties>>;
 type GateMapState = Vec<(String, Vec<(Option<Qargs>, Option<InstructionProperties>)>)>;
 
+/// Represents a Qiskit `Gate` object or a Variadic instruction.
+/// Keeps a reference to its Python instance for caching purposes.
 #[derive(Debug, Clone, FromPyObject)]
 pub(crate) enum TargetOperation {
     Normal(NormalOperation),
@@ -96,6 +99,8 @@ impl TargetOperation {
     }
 }
 
+/// Represents a Qiskit `Gate` object, keeps a reference to its Python
+/// instance for caching purposes.
 #[derive(Debug, Clone)]
 pub(crate) struct NormalOperation {
     pub operation: OperationType,
@@ -133,6 +138,10 @@ constraints of a particular backend.
 The intent of this struct is to contain data that can be representable and
 accessible through both Rust and Python, so it can be used for rust-based
 transpiler processes.
+
+This structure contains duplicates of every element in the Python counterpart of
+`gate_map`. Which improves access for Python while sacrificing a small amount of
+memory.
  */
 #[pyclass(
     mapping,
@@ -1251,3 +1260,5 @@ pub fn target(_py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<Target>()?;
     Ok(())
 }
+
+// TODO: Add rust-based unit testing.

qiskit/transpiler/target.py

@@ -746,9 +746,7 @@ def _build_coupling_graph(self):
             for qarg, properties in qarg_map.items():
                 if qarg is None:
                     if self.operation_from_name(gate).num_qubits == 2:
-                        self._coupling_graph = (
-                            None  # pylint: disable=attribute-defined-outside-init
-                        )
+                        self._coupling_graph = None
                         return
                     continue
                 if len(qarg) == 1:
@@ -893,12 +891,10 @@ def __str__(self):
                 prop_str_pieces = [f"\t\t{qarg}:\n"]
                 duration = getattr(props, "duration", None)
                 if duration is not None:
-                    prop_str_pieces.append(
-                        f"\t\t\tDuration: {duration if duration > 0 else 0} sec.\n"
-                    )
+                    prop_str_pieces.append(f"\t\t\tDuration: {duration:g} sec.\n")
                 error = getattr(props, "error", None)
                 if error is not None:
-                    prop_str_pieces.append(f"\t\t\tError Rate: {error if error > 0 else 0}\n")
+                    prop_str_pieces.append(f"\t\t\tError Rate: {error:g}\n")
                 schedule = getattr(props, "_calibration", None)
                 if schedule is not None:
                     prop_str_pieces.append("\t\t\tWith pulse schedule calibration\n")