arg_validator: support generating default value for ArgValidatorDict

Some dictionaries may wish to have a default value that is a dictionary
with the default of all keys.

Add a method to generate this.

This returns a callable instead of a plain dictionary. In practice, the
generated default value should always be the same. However, we also
expect that ArgValidator validation gives a result that is owned by the
caller. In case of mutable values (like a dictionary), we want a deep
clone.

Signed-off-by: Thomas Haller <thaller@redhat.com>

module_utils/network_lsr/argument_validator.py

@@ -337,11 +337,14 @@ class ArgValidatorDict(ArgValidator):
         default_value=None,
         all_missing_during_validate=False,
     ):
-        ArgValidator.__init__(self, name, required, default_value)
         if nested is not None:
-            self.nested = dict([(v.name, v) for v in nested])
+            nested = dict([(v.name, v) for v in nested])
         else:
-            self.nested = {}
+            nested = {}
+        if default_value is ArgValidator.DEFAULT:
+            default_value = self.generate_default
+        ArgValidator.__init__(self, name, required, default_value)
+        self.nested = nested
         self.all_missing_during_validate = all_missing_during_validate
 
     def _validate_impl(self, value, name):
@@ -378,6 +381,18 @@ class ArgValidatorDict(ArgValidator):
                     result[setting] = default
         return result
 
+    @staticmethod
+    def generate_default_from_nested(nested):
+        result = {}
+        for name, validator in nested.items():
+            default = validator.get_default_value()
+            if default is not ArgValidator.MISSING:
+                result[name] = default
+        return result
+
+    def generate_default(self):
+        return ArgValidatorDict.generate_default_from_nested(self.nested)
+
 
 class ArgValidatorList(ArgValidator):
     def __init__(self, name, nested, default_value=None):