Remove method qasm from Instruction and all of its subclasses

qiskit/circuit/instruction.py

@@ -43,6 +43,7 @@
 from qiskit.qobj.qasm_qobj import QasmQobjInstruction
 from qiskit.circuit.parameter import ParameterExpression
 from qiskit.circuit.operation import Operation
+from qiskit.utils.deprecation import deprecate_func
 from .tools import pi_check
 
 _CUTOFF_PRECISION = 1e-10
@@ -447,6 +448,14 @@ def _qasmif(self, string):
             )
         return "if(%s==%d) " % (self.condition[0].name, self.condition[1]) + string
 
+    @deprecate_func(
+        additional_msg=(
+            "Correct exporting to OpenQASM 2 is the responsibility of a larger exporter; it cannot "
+            "safely be done on an object-by-object basis without context. No replacement will be "
+            "provided, because the premise is wrong."
+        ),
+        since="0.25.0",
+    )
     def qasm(self):
         """Return a default OpenQASM string for the instruction.
 

qiskit/circuit/library/standard_gates/x.py

@@ -15,6 +15,7 @@
 from typing import Optional, Union, Type
 from math import ceil, pi
 import numpy
+from qiskit.utils.deprecation import deprecate_func
 from qiskit.circuit.controlledgate import ControlledGate
 from qiskit.circuit.gate import Gate
 from qiskit.circuit.quantumregister import QuantumRegister
@@ -568,6 +569,7 @@ def _define(self):
 
         self.definition = qc
 
+    @deprecate_func(since="0.25.0")
     def qasm(self):
         # Gross hack to override the Qiskit name with the name this gate has in Terra's version of
         # 'qelib1.inc'.  In general, the larger exporter mechanism should know about this to do the

qiskit/circuit/quantumcircuit.py

@@ -44,6 +44,7 @@
 from qiskit.exceptions import QiskitError
 from qiskit.utils.multiprocessing import is_main_process
 from qiskit.circuit.instruction import Instruction
+import qiskit.circuit.instruction
 from qiskit.circuit.gate import Gate
 from qiskit.circuit.parameter import Parameter
 from qiskit.circuit.exceptions import CircuitError
@@ -62,6 +63,7 @@
 from .delay import Delay
 from .measure import Measure
 from .reset import Reset
+from .tools import pi_check
 
 if typing.TYPE_CHECKING:
     import qiskit  # pylint: disable=cyclic-import
@@ -1646,6 +1648,7 @@ def qasm(
             MissingOptionalLibraryError: If pygments is not installed and ``formatted`` is
                 ``True``.
             QASM2ExportError: If circuit has free parameters.
+            QASM2ExportError: If an operation that has no OpenQASM 2 representation is encountered.
         """
         from qiskit.qasm2 import QASM2ExportError  # pylint: disable=cyclic-import
 
@@ -1741,15 +1744,9 @@ def qasm(
                 qargs = ",".join(bit_labels[q] for q in instruction.qubits)
                 instruction_qasm = "barrier;" if not qargs else f"barrier {qargs};"
             else:
-                operation = _qasm2_define_custom_operation(
-                    operation, existing_gate_names, gates_to_define
+                instruction_qasm = _qasm2_custom_operation_statement(
+                    instruction, existing_gate_names, gates_to_define, bit_labels
                 )
-
-                # Insert qasm representation of the original instruction
-                bits_qasm = ",".join(
-                    bit_labels[j] for j in itertools.chain(instruction.qubits, instruction.clbits)
-                )
-                instruction_qasm = f"{operation.qasm()} {bits_qasm};"
             instruction_calls.append(instruction_qasm)
         instructions_qasm = "".join(f"{call}\n" for call in instruction_calls)
         gate_definitions_qasm = "".join(f"{qasm}\n" for _, qasm in gates_to_define.values())
@@ -5022,6 +5019,22 @@ def _compare_parameters(param1: Parameter, param2: Parameter) -> int:
 _QASM2_FIXED_PARAMETERS = [Parameter("param0"), Parameter("param1"), Parameter("param2")]
 
 
+def _qasm2_custom_operation_statement(
+    instruction, existing_gate_names, gates_to_define, bit_labels
+):
+    operation = _qasm2_define_custom_operation(
+        instruction.operation, existing_gate_names, gates_to_define
+    )
+    # Insert qasm representation of the original instruction
+    if instruction.clbits:
+        bits = itertools.chain(instruction.qubits, instruction.clbits)
+    else:
+        bits = instruction.qubits
+    bits_qasm = ",".join(bit_labels[j] for j in bits)
+    instruction_qasm = f"{_instruction_qasm2(operation)} {bits_qasm};"
+    return instruction_qasm
+
+
 def _qasm2_define_custom_operation(operation, existing_gate_names, gates_to_define):
     """Extract a custom definition from the given operation, and append any necessary additional
     subcomponents' definitions to the ``gates_to_define`` ordered dictionary.
@@ -5097,15 +5110,13 @@ def _qasm2_define_custom_operation(operation, existing_gate_names, gates_to_defi
             f"opaque {new_name}{parameters_qasm} {qubits_qasm};",
         )
     else:
-        statements = []
         qubit_labels = {bit: f"q{i}" for i, bit in enumerate(parameterized_definition.qubits)}
-        for instruction in parameterized_definition.data:
-            new_operation = _qasm2_define_custom_operation(
-                instruction.operation, existing_gate_names, gates_to_define
+        body_qasm = " ".join(
+            _qasm2_custom_operation_statement(
+                instruction, existing_gate_names, gates_to_define, qubit_labels
             )
-            bits_qasm = ",".join(qubit_labels[q] for q in instruction.qubits)
-            statements.append(f"{new_operation.qasm()} {bits_qasm};")
-        body_qasm = " ".join(statements)
+            for instruction in parameterized_definition.data
+        )
 
         # if an inner operation has the same name as the actual operation, it needs to be renamed
         if operation.name in gates_to_define:
@@ -5138,6 +5149,30 @@ def _qasm_escape_name(name: str, prefix: str) -> str:
     return escaped_name
 
 
+def _instruction_qasm2(operation):
+    """Return an OpenQASM 2 string for the instruction."""
+    from qiskit.qasm2 import QASM2ExportError  # pylint: disable=cyclic-import
+
+    if operation.name == "c3sx":
+        qasm2_call = "c3sqrtx"
+    else:
+        qasm2_call = operation.name
+    if operation.params:
+        qasm2_call = "{}({})".format(
+            qasm2_call,
+            ",".join([pi_check(i, output="qasm", eps=1e-12) for i in operation.params]),
+        )
+    if operation.condition is not None:
+        if not isinstance(operation.condition[0], ClassicalRegister):
+            raise QASM2ExportError(
+                "OpenQASM 2 can only condition on registers, but got '{operation.condition[0]}'"
+            )
+        qasm2_call = (
+            "if(%s==%d) " % (operation.condition[0].name, operation.condition[1]) + qasm2_call
+        )
+    return qasm2_call
+
+
 def _make_unique(name: str, already_defined: collections.abc.Set[str]) -> str:
     """Generate a name by suffixing the given stem that is unique within the defined set."""
     if name not in already_defined:

qiskit/extensions/hamiltonian_gate.py

@@ -22,6 +22,7 @@
 from qiskit.quantum_info.operators.predicates import is_hermitian_matrix
 from qiskit.extensions.exceptions import ExtensionError
 from qiskit.circuit.exceptions import CircuitError
+from qiskit.utils.deprecation import deprecate_func
 
 from .unitary import UnitaryGate
 
@@ -116,6 +117,9 @@ def _define(self):
         qc._append(UnitaryGate(self.to_matrix()), q[:], [])
         self.definition = qc
 
+    @deprecate_func(
+        since="0.25.0",
+    )
     def qasm(self):
         """Raise an error, as QASM is not defined for the HamiltonianGate."""
         raise ExtensionError("HamiltonianGate has no QASM definition.")

releasenotes/notes/deprecate-instruction-qasm-9380f721e7bdaf6b.yaml

@@ -0,0 +1,6 @@
+---
+deprecations:
+  - |
+    The method :meth:`~qiskit.circuit.Instruction.qasm` and all overriding methods of subclasses
+    of `:meth:`~qiskit.circuit.Instruction` are deprecated. There is no replacement for generating
+    an OpenQASM 2 string for an isolated instruction.

test/python/circuit/test_circuit_qasm.py

@@ -266,6 +266,11 @@ def test_c3sxgate_roundtrips(self):
         parsed = QuantumCircuit.from_qasm_str(qasm)
         self.assertIsInstance(parsed.data[0].operation, C3SXGate)
 
+    def test_c3sxgate_qasm_deprecation_warning(self):
+        """Test deprecation warning for C3SXGate."""
+        with self.assertWarnsRegex(DeprecationWarning, r"Correct exporting to OpenQASM 2"):
+            C3SXGate().qasm()
+
     def test_cczgate_qasm(self):
         """Test that CCZ dumps definition as a non-qelib1 gate."""
         qc = QuantumCircuit(3)

test/python/circuit/test_hamiltonian_gate.py

@@ -89,16 +89,13 @@ def test_1q_hamiltonian(self):
         self.assertEqual(dnode.qargs, tuple(qc.qubits))
         assert_allclose(dnode.op.to_matrix(), np.eye(2))
 
-    def test_error_on_qasm(self):
-        """test that an error is thrown if qc.qasm() is called."""
-        qr = QuantumRegister(1, "q0")
-        cr = ClassicalRegister(1, "c0")
-        qc = QuantumCircuit(qr, cr)
+    def test_error_and_deprecation_warning_on_qasm(self):
+        """test that an error is thrown if the method `qasm` is called."""
         matrix = np.zeros((2, 2))
-        qc.hamiltonian(operator=matrix, time=1, qubits=qr[0])
-
+        hamiltonian_gate = HamiltonianGate(data=matrix, time=1)
         with self.assertRaises(ExtensionError):
-            qc.qasm()
+            with self.assertWarns(DeprecationWarning):
+                hamiltonian_gate.qasm()
 
     def test_2q_hamiltonian(self):
         """test 2 qubit hamiltonian"""

test/python/circuit/test_instructions.py

@@ -628,6 +628,15 @@ def test_label_type_enforcement(self):
                 instruction = HGate()
                 instruction.label = 0
 
+    def test_deprecation_warnings_qasm_methods(self):
+        """Test deprecation warnings for qasm methods."""
+        with self.subTest("built in gates"):
+            with self.assertWarnsRegex(DeprecationWarning, r"Correct exporting to OpenQASM 2"):
+                HGate().qasm()
+        with self.subTest("User constructed Instruction"):
+            with self.assertWarnsRegex(DeprecationWarning, r"Correct exporting to OpenQASM 2"):
+                Instruction("v", 1, 0, [0.4, 0.5, 0.5]).qasm()
+
 
 if __name__ == "__main__":
     unittest.main()