RF: Disallow overwriting config settings via python attribute assignment

bip/utils/config.py

@@ -56,6 +56,12 @@ specifying a directory.
 """
 
 
+class ConfigError(Exception):
+    """Custom Exception class used by the Config class.  Raised by
+    Config.__setattr__ on attempts to overwrite a configuration setting.
+    """
+
+
 def nested_lookup(d, key):
     """Look up a value in a nested dictionary based on the given key.
     For example, imagine we have a dictionary d:
@@ -443,7 +449,20 @@ class Config:
 
     def __getattr__(self, key):
         """Return the configuration item with the specified key. """
-        return self[key]
+        try:
+            return self[key]
+        except KeyError:
+            raise AttributeError(key)
+
+
+    def __setattr__(self, key, value):
+        """Raise an error on attempts to modify a setting value. """
+        # Ignore the initial self.__settings assignment,
+        # otherwise we infinitely recurse.
+        if key != '_Config__settings' and key in self:
+            raise ConfigError(f'Attempt to overwrite config value '
+                              f'{key} (value: {value})')
+        super().__setattr__(key, value)
 
 
     def get(self, key, default=None):