driver/pyvisadriver: add optional backend property

Allow users to override PyVISA backed to use something like PyVISA-sim.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>

Author: liambeguin

doc/configuration.rst

@@ -1160,6 +1160,7 @@ Arguments:
     ASRL, TCPIP...
   - url (str): device identifier on selected resource, e.g. <ip> for TCPIP
     resource
+  - backend (str): Visa library backend, e.g. '@sim' for pyvisa-sim backend
 
 Used by:
   - `PyVISADriver`_

labgrid/driver/pyvisadriver.py

@@ -18,7 +18,7 @@ class PyVISADriver(Driver):
     def __attrs_post_init__(self):
         super().__attrs_post_init__()
         _py_pyvisa_module = import_module('pyvisa')
-        self._pyvisa_resource_manager = _py_pyvisa_module.ResourceManager()
+        self._pyvisa_resource_manager = _py_pyvisa_module.ResourceManager(self.pyvisa_resource.backend)
         self.pyvisa_device = None
 
     def on_activate(self):

labgrid/resource/pyvisa.py

@@ -12,6 +12,8 @@ class PyVISADevice(Resource):
     Args:
         type (str): device resource type following the pyVISA resource syntax, e.g. ASRL, TCPIP...
         url (str): device identifier on selected resource, e.g. <ip> for TCPIP resource
+        backend (str, default=''): Visa library backend, e.g. '@sim' for pyvisa-sim backend
     """
     type = attr.ib(validator=attr.validators.instance_of(str))
     url = attr.ib(validator=attr.validators.instance_of(str))
+    backend = attr.ib(default='', validator=attr.validators.instance_of(str))