Instr param (microsoft#649)

* added instrument par

* added test

* Addressed comments

qcodes/instrument/parameter.py

@@ -67,7 +67,7 @@
                                   full_class, named_repr, warn_units)
 from qcodes.utils.metadata import Metadatable
 from qcodes.utils.command import Command, NoCommandError
-from qcodes.utils.validators import Validator, Numbers, Ints, Enum
+from qcodes.utils.validators import Validator, Numbers, Ints, Enum, Strings
 from qcodes.instrument.sweep_values import SweepFixedValues
 from qcodes.data.data_array import DataArray
 
@@ -1073,9 +1073,9 @@ class ManualParameter(Parameter):
         instrument (Optional[Instrument]): the instrument this applies to,
             if any.
 
-        initial_value (Optional[str]): starting value, the
-            only invalid value allowed, and None is only allowed as an initial
-            value, it cannot be set later
+        initial_value (Optional[str]): starting value, may be None even if
+            None does not pass the validator. None is only allowed as an
+            initial value and cannot be set after initiation.
 
         **kwargs: Passed to Parameter parent class
     """
@@ -1288,3 +1288,50 @@ def snapshot_base(self, update=False):
             meta_data[param.full_name] = param.snapshot()
 
         return meta_data
+
+
+class InstrumentRefParameter(ManualParameter):
+    """
+    An InstrumentRefParameter
+
+    Args:
+        name (string): the name of the parameter that one wants to add.
+
+        instrument (Optional[Instrument]): the "parent" instrument this
+            parameter is attached to, if any.
+
+        initial_value (Optional[str]): starting value, may be None even if
+            None does not pass the validator. None is only allowed as an
+            initial value and cannot be set after initiation.
+
+        **kwargs: Passed to InstrumentRefParameter parent class
+
+    This parameter is useful when one needs a reference to another instrument
+    from within an instrument, e.g., when creating a meta instrument that
+    sets parameters on instruments it contains.
+    """
+
+    def get_instr(self):
+        """
+        Returns the instance of the instrument with the name equal to the
+        value of this parameter.
+        """
+        ref_instrument_name = self.get()
+        # note that _instrument refers to the instrument this parameter belongs
+        # to, while the ref_instrument_name is the instrument that is the value
+        # of this parameter.
+        return self._instrument.find_instrument(ref_instrument_name)
+
+    def set_validator(self, vals):
+        """
+        Set a validator `vals` for this parameter.
+
+        Args:
+            vals (Validator):  validator to set
+        """
+        if vals is None:
+            self._vals = Strings()
+        elif isinstance(vals, Validator):
+            self._vals = vals
+        else:
+            raise TypeError('vals must be a Validator')

qcodes/tests/test_parameter.py

@@ -7,9 +7,10 @@
 from qcodes import Function
 from qcodes.instrument.parameter import (
     Parameter, ArrayParameter, MultiParameter,
-    ManualParameter, StandardParameter)
+    ManualParameter, StandardParameter, InstrumentRefParameter)
 from qcodes.utils.helpers import LogCapture
 from qcodes.utils.validators import Numbers
+from qcodes.tests.instrument_mocks import DummyInstrument
 
 
 class GettableParam(Parameter):
@@ -636,3 +637,15 @@ def test_val_mapping_with_parsers(self):
 
         self._p = 'PVAL: 1'
         self.assertEqual(p(), 'on')
+
+
+class TestInstrumentRefParameter(TestCase):
+    def test_get_instr(self):
+        a = DummyInstrument('dummy_holder')
+        d = DummyInstrument('dummy')
+        a.add_parameter('test', parameter_class=InstrumentRefParameter)
+
+        a.test.set(d.name)
+
+        self.assertEqual(a.test.get(), d.name)
+        self.assertEqual(a.test.get_instr(), d)