fix(instrument): silence get_idn warning on virtual instruments

docs/changes/newsfragments/4611.improved

@@ -0,0 +1,4 @@
+:class:`~qcodes.instrument.VirtualInstrument` has been added for virtual drivers
+that do not communicate with hardware. Its ``get_idn`` returns a default IDN
+dict without warning. :meth:`~qcodes.instrument.Instrument.get_idn` remains
+unchanged and still warns when communication via ``ask_raw`` is unavailable.

src/qcodes/__init__.py

@@ -51,6 +51,7 @@
     Instrument,
     InstrumentChannel,
     IPInstrument,
+    VirtualInstrument,
     VisaInstrument,
     find_or_create_instrument,
 )

src/qcodes/instrument/__init__.py

@@ -16,7 +16,7 @@
 )
 
 from .channel import ChannelList, ChannelTuple, InstrumentChannel, InstrumentModule
-from .instrument import Instrument, find_or_create_instrument
+from .instrument import Instrument, VirtualInstrument, find_or_create_instrument
 from .instrument_base import InstrumentBase, InstrumentBaseKWArgs
 from .ip import IPInstrument
 from .visa import VisaInstrument, VisaInstrumentKWArgs
@@ -30,6 +30,7 @@
     "InstrumentBaseKWArgs",
     "InstrumentChannel",
     "InstrumentModule",
+    "VirtualInstrument",
     "VisaInstrument",
     "VisaInstrumentKWArgs",
     "find_or_create_instrument",

src/qcodes/instrument/instrument.py

@@ -461,6 +461,15 @@ def ask_raw(self, cmd: str) -> str:
         )
 
 
+class VirtualInstrument(Instrument):
+    """
+    Base class for virtual instruments that do not communicate with hardware.
+    """
+
+    def get_idn(self) -> dict[str, str | None]:
+        return {"vendor": None, "model": self.name, "serial": None, "firmware": None}
+
+
 def find_or_create_instrument(
     instrument_class: type[T],
     name: str,

tests/test_instrument.py

@@ -19,6 +19,7 @@
     Instrument,
     InstrumentBase,
     InstrumentModule,
+    VirtualInstrument,
     find_or_create_instrument,
 )
 from qcodes.instrument_drivers.mock_instruments import (
@@ -256,6 +257,74 @@ def test_get_idn(testdummy: DummyInstrument) -> None:
     assert testdummy.get_idn() == idn
 
 
+def test_get_idn_on_virtual_instrument_does_not_warn(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """``VirtualInstrument.get_idn`` should return a default IDN without warning."""
+
+    virtual = VirtualInstrument(name="virtual_no_ask")
+    try:
+        caplog.clear()
+        with caplog.at_level("WARNING"):
+            idn = virtual.get_idn()
+        assert idn == {
+            "vendor": None,
+            "model": "virtual_no_ask",
+            "serial": None,
+            "firmware": None,
+        }
+        assert "Error getting or interpreting *IDN?" not in caplog.text
+    finally:
+        virtual.close()
+
+
+def test_get_idn_on_base_instrument_warns(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """The plain ``Instrument`` base class should still warn without ``ask_raw``."""
+
+    instrument = Instrument(name="instrument_no_ask")
+    try:
+        caplog.clear()
+        with caplog.at_level("WARNING"):
+            idn = instrument.get_idn()
+        assert idn == {
+            "vendor": None,
+            "model": "instrument_no_ask",
+            "serial": None,
+            "firmware": None,
+        }
+        assert "Error getting or interpreting *IDN?" in caplog.text
+    finally:
+        instrument.close()
+
+
+def test_get_idn_still_warns_on_other_errors(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """Unexpected errors in ``*IDN?`` handling should still be surfaced via a
+    warning so real misbehaviour is not silenced by the virtual-instrument
+    short-circuit."""
+
+    class BrokenInstrument(Instrument):
+        def ask_raw(self, cmd: str) -> str:
+            raise RuntimeError("communication failure")
+
+    broken = BrokenInstrument(name="broken_ask")
+    try:
+        with caplog.at_level("WARNING"):
+            idn = broken.get_idn()
+        assert idn == {
+            "vendor": None,
+            "model": "broken_ask",
+            "serial": None,
+            "firmware": None,
+        }
+        assert "Error getting or interpreting *IDN?" in caplog.text
+    finally:
+        broken.close()
+
+
 def test_repr(testdummy: DummyInstrument) -> None:
     assert repr(testdummy) == "<DummyInstrument: testdummy>"