def test_qft_mutability(self):
"""Test the mutability of the QFT circuit."""
qft = QFT()
with self.subTest(msg='empty initialization'):
self.assertEqual(qft.num_qubits, 0)
self.assertEqual(qft.data, [])
with self.subTest(msg='changing number of qubits'):
qft.num_qubits = 3
self.assertQFTIsCorrect(qft, num_qubits=3)
with self.subTest(msg='test diminishing the number of qubits'):
qft.num_qubits = 1
self.assertQFTIsCorrect(qft, num_qubits=1)
with self.subTest(msg='test with swaps'):
qft.num_qubits = 4
qft.do_swaps = False
self.assertQFTIsCorrect(qft, add_swaps_at_end=True)
with self.subTest(msg='set approximation'):
qft.approximation_degree = 2
qft.do_swaps = True
with self.assertRaises(AssertionError):
self.assertQFTIsCorrect(qft)
def test_warns_if_too_large(self):
"""Test that a warning is issued if the user tries to make a circuit that would need to
represent angles smaller than the smallest normal double-precision floating-point number.
It's too slow to actually let QFT construct a 1050+ qubit circuit for such a simple test, so
we temporarily prevent QuantumCircuits from being created in order to short-circuit the QFT
builder."""
class SentinelException(Exception):
"""Dummy exception that raises itself as soon as it is created."""
def __init__(self, *_args, **_kwargs):
super().__init__()
raise self
# We don't want to issue a warning on mutation until we know that the values are
# finalised; this is because a user might want to mutate the number of qubits and the
# approximation degree. In these cases, wait until we try to build the circuit.
with warnings.catch_warnings(record=True) as caught_warnings:
warnings.filterwarnings(
"always",
category=RuntimeWarning,
module=r"qiskit\..*",
message=r".*precision loss in QFT.*",
)
qft = QFT()
# Even with the approximation this will trigger the warning.
qft.num_qubits = 1080
qft.approximation_degree = 20
self.assertFalse(caught_warnings)
# Short-circuit the build method so it exits after input validation, but without actually
# spinning the CPU to build a huge, useless object.
with unittest.mock.patch("qiskit.circuit.QuantumCircuit.__init__", SentinelException):
with self.assertWarnsRegex(RuntimeWarning, "precision loss in QFT"):
with self.assertRaises(SentinelException):
qft._build()
