Add control comms

shepherd/config.py

@@ -81,7 +81,7 @@ class IntDef(_ConfigDefinition):
 
 @freezedryable
 class StringDef(_ConfigDefinition):
-    def __init__(self, default=None, minlength=None, maxlength=None,
+    def __init__(self, default="", minlength=None, maxlength=None,
                  optional=False, helptext=""):
         super().__init__(default, optional, helptext)
         self.minlength = minlength
@@ -89,7 +89,7 @@ class StringDef(_ConfigDefinition):
 
     def validate(self, value):
         if not isinstance(value, str):
-            raise InvalidConfigError("Config value must be a string")
+            raise InvalidConfigError(F"Config value must be a string and is {value}")
         if self.minlength is not None and len(value) < self.minlength:
             raise InvalidConfigError("Config string length must be >= " +
                                      str(self.minlength))