Setting qutip-qip as an optional plugin for qutip-5 [unitaryhack]

qutip/qip.py

@@ -0,0 +1,13 @@
+"""Module replicating the qutip_qip package from within qutip."""
+import sys
+
+try:
+    import qutip_qip
+    del qutip_qip
+    sys.modules["qutip.qip"] = sys.modules["qutip_qip"]
+except ImportError:
+    raise ImportError(
+        "Importing 'qutip.qip' requires the 'qutip_qip' package. Install it "
+        "with `pip install qutip-qip` (for more details, go to "
+        "https://qutip-qip.rtfd.io/en/stable/)."
+    )

qutip/tests/test_qip.py

@@ -0,0 +1,45 @@
+import sys
+
+import pytest
+
+
+class QutipQipStub:
+    class CircuitModuleStub:
+        QubitCircuit = "FakeQubitCircuit"
+
+    circuit = CircuitModuleStub()
+
+
+@pytest.fixture
+def without_qutip_qip(monkeypatch):
+    monkeypatch.setitem(sys.modules, "qutip_qip", None)
+    monkeypatch.delitem(sys.modules, "qutip.qip", raising=False)
+
+
+@pytest.fixture
+def with_qutip_qip_stub(monkeypatch):
+    monkeypatch.setitem(sys.modules, "qutip_qip", QutipQipStub())
+    monkeypatch.delitem(sys.modules, "qutip.qip", raising=False)
+
+
+def test_failed_import(without_qutip_qip):
+    # Ensure 'qutip.qip' is not imported yet
+    assert "qutip.qip" not in sys.modules
+    with pytest.raises(
+        ImportError,
+        match="Importing 'qutip.qip' requires the 'qutip_qip' package.",
+    ):
+        import qutip.qip
+
+
+def test_with_qip(monkeypatch, with_qutip_qip_stub):
+    import qutip.qip
+
+    monkeypatch.setitem(sys.modules, "qutip.qip.circuit", qutip.qip.circuit)
+    import qutip.qip.circuit as circuit
+    from qutip.qip.circuit import QubitCircuit
+    import qutip_qip
+
+    assert qutip.qip is qutip_qip
+    assert circuit is qutip_qip.circuit
+    assert QubitCircuit is qutip_qip.circuit.QubitCircuit