Remove method qasm from Instruction and all of its subclasses

qiskit/circuit/instruction.py

@@ -44,6 +44,7 @@
 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
@@ -446,6 +447,14 @@ 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 +569,14 @@ def _define(self):
 
         self.definition = qc
 
+    @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):
         # 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, MissingOptionalLibraryError
 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.qasm.exceptions import QasmError
@@ -61,6 +62,7 @@
 from .delay import Delay
 from .measure import Measure
 from .reset import Reset
+from .tools import pi_check
 
 try:
     import pygments
@@ -1646,6 +1648,7 @@ def qasm(
             MissingOptionalLibraryError: If pygments is not installed and ``formatted`` is
                 ``True``.
             QasmError: If circuit has free parameters.
+            CircuitError: If an operation that has no OpenQASM 2 representation is encountered.
         """
 
         if self.num_parameters > 0:
@@ -1746,7 +1749,7 @@ def qasm(
                 bits_qasm = ",".join(
                     bit_labels[j] for j in itertools.chain(instruction.qubits, instruction.clbits)
                 )
-                instruction_qasm = f"{operation.qasm()} {bits_qasm};"
+                instruction_qasm = f"{_instruction_qasm2(operation)} {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())
@@ -5085,7 +5088,7 @@ def _qasm2_define_custom_operation(operation, existing_gate_names, gates_to_defi
                 instruction.operation, existing_gate_names, gates_to_define
             )
             bits_qasm = ",".join(qubit_labels[q] for q in instruction.qubits)
-            statements.append(f"{new_operation.qasm()} {bits_qasm};")
+            statements.append(f"{_instruction_qasm2(new_operation)} {bits_qasm};")
         body_qasm = " ".join(statements)
 
         # if an inner operation has the same name as the actual operation, it needs to be renamed
@@ -5199,3 +5202,25 @@ def _bit_argument_conversion_scalar(specifier, bit_sequence, bit_set, type_):
         else f"Invalid bit index: '{specifier}' of type '{type(specifier)}'"
     )
     raise CircuitError(message)
+
+
+def _instruction_qasm2(operation):
+    """Return an OpenQASM 2 string for the instruction."""
+    if operation.name == "c3sx":
+        name_param = "c3sqrtx"
+    else:
+        name_param = operation.name
+    if operation.params:
+        name_param = "{}({})".format(
+            name_param,
+            ",".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 QasmError(
+                "OpenQASM 2 can only condition on registers, but got '{operation.condition[0]}'"
+            )
+        name_param = (
+            "if(%s==%d) " % (operation.condition[0].name, operation.condition[1]) + name_param
+        )
+    return name_param

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()