make instrument optional in Function too

qcodes/instrument/base.py

@@ -64,7 +64,7 @@ def add_function(self, name, **kwargs):
         '''
         if name in self.functions:
             raise KeyError('Duplicate function name {}'.format(name))
-        self.functions[name] = Function(self, name, **kwargs)
+        self.functions[name] = Function(name=name, instrument=self, **kwargs)
 
     def snapshot_base(self, update=False):
         if update:

qcodes/instrument/function.py

@@ -6,41 +6,45 @@
 
 
 class Function(Metadatable):
-    def __init__(self, instrument, name, call_cmd=None, async_call_cmd=None,
+    '''
+    defines a function (with arbitrary parameters) that this instrument
+    can execute.
+
+    You execute this function object like a normal function, or use its
+    .call method; or call it async with the .call_async method.
+
+    name: the local name of this parameter
+    instrument: an instrument that handles this function
+        default None
+
+    call_cmd: command to execute on instrument
+        - a string (with positional fields to .format, "{}" or "{0}" etc)
+          you can only use a string if an instrument is provided,
+          this string will be passed to instrument.write
+        - a function (with parameter count matching parameters list)
+    async_call_cmd: an async function to use for call_async, or for both
+        sync and async if call_cmd is missing or None.
+
+    parameters: list of Validator objects,
+        one for each parameter to the Function
+
+    parameter_parser: function to transform the input parameter(s)
+        to encoded value(s) sent to the instrument.
+        If there are multiple arguments, this function should accept all
+        the arguments in order, and return a tuple of values.
+    return_parser: function to transform the response from the instrument
+        to the final output value.
+        may be a type casting function like `int` or `float`.
+        If None (default), will not wait for or read any response
+    NOTE: parsers only apply if call_cmd is a string. The function forms
+        of call_cmd and async_call_cmd should do their own parsing.
+
+    parse_function: DEPRECATED - use return_parser instead
+    '''
+    def __init__(self, name, instrument=None,
+                 call_cmd=None, async_call_cmd=None,
                  parameters=[], parameter_parser=None, return_parser=None,
                  parse_function=None, **kwargs):
-        '''
-        defines a function (with arbitrary parameters) that this instrument
-        can execute.
-
-        You execute this function object like a normal function, or use its
-        .call method; or call it async with the .call_async method.
-
-        instrument: an instrument that handles this function
-        name: the local name of this parameter
-
-        call_cmd: command to execute on instrument
-            - a string (with positional fields to .format, "{}" or "{0}" etc)
-            - a function (with parameter count matching parameters list)
-        async_call_cmd: an async function to use for call_async, or for both
-            sync and async if call_cmd is missing or None.
-
-        parameters: list of Validator objects,
-            one for each parameter to the Function
-
-        parameter_parser: function to transform the input parameter(s)
-            to encoded value(s) sent to the instrument.
-            If there are multiple arguments, this function should accept all
-            the arguments in order, and return a tuple of values.
-        return_parser: function to transform the response from the instrument
-            to the final output value.
-            may be a type casting function like `int` or `float`.
-            If None (default), will not wait for or read any response
-        NOTE: parsers only apply if call_cmd is a string. The function forms
-            of call_cmd and async_call_cmd should do their own parsing.
-
-        parse_function: DEPRECATED - use return_parser instead
-        '''
         super().__init__(**kwargs)
 
         self._instrument = instrument
@@ -63,11 +67,14 @@ def _set_params(self, parameters):
 
     def _set_call(self, call_cmd, async_call_cmd,
                   parameter_parser, return_parser):
-        ask_or_write = self._instrument.write
-        ask_or_write_async = self._instrument.write_async
-        if isinstance(call_cmd, str) and return_parser:
-            ask_or_write = self._instrument.ask
-            ask_or_write_async = self._instrument.ask_async
+        if self._instrument:
+            ask_or_write = self._instrument.write
+            ask_or_write_async = self._instrument.write_async
+            if isinstance(call_cmd, str) and return_parser:
+                ask_or_write = self._instrument.ask
+                ask_or_write_async = self._instrument.ask_async
+        else:
+            ask_or_write, ask_or_write_async = None, None
 
         self._call, self._call_async = syncable_command(
             param_count=self._param_count,