Config: Remove use of NO_DEFAULT for Option.default

The `Option.default` property would return the global constant
`NO_DEFAULT` in case the option does not specify a default. The idea was
that it could be used to distinguish between an option not defining a
default and one defining the default to be `None`.

The problem is that in the various methods that return a config option
value, such as `Config.get_option` could also return this value. This
would be problematic for certain CLI command options that used the
`aiida.manage.configuration.get_config_option` function to set the
default. If the config option was not defined, the function would return
`()` which is the value of the `NO_DEFAULT` constant. When the option
accepts string values, this value would often even silently be accepted
although it almost certainly is not what the user intended.

This would actually happen for the tests of `verdi setup`, which has the
options `--email`, `--first-name`, `--last-name` and `--institution`
that all define a default through `get_config_option` and therefore the
default would be actually set to `()` in case the config did not specify
these global config options.

Since for config options there is no current use-case for actually
setting a default to `None`, there is no need to distinguish between
this case and a default never having been defined, and so the `NO_DEFAULT`
global constant is removed and replaced by `None`.

aiida/cmdline/commands/cmd_config.py

@@ -77,15 +77,14 @@ def _join(val):
 def verdi_config_show(ctx, option):
     """Show details of an AiiDA option for the current profile."""
     from aiida.manage.configuration import Config, Profile
-    from aiida.manage.configuration.options import NO_DEFAULT
 
     config: Config = ctx.obj.config
     profile: Profile | None = ctx.obj.profile
 
     dct = {
         'schema': option.schema,
         'values': {
-            'default': '<NOTSET>' if option.default is NO_DEFAULT else option.default,
+            'default': '<NOTSET>' if option.default is None else option.default,
             'global': config.options.get(option.name, '<NOTSET>'),
         }
     }

aiida/manage/configuration/config.py

@@ -20,7 +20,7 @@
 from aiida.common.exceptions import ConfigurationError
 
 from . import schema as schema_module
-from .options import NO_DEFAULT, Option, get_option, get_option_names, parse_option
+from .options import Option, get_option, get_option_names, parse_option
 from .profile import Profile
 
 __all__ = ('Config', 'config_schema', 'ConfigValidationError')
@@ -407,7 +407,7 @@ def set_option(self, option_name, option_value, scope=None, override=True):
 
         if parsed_value is not None:
             value = parsed_value
-        elif option.default is not NO_DEFAULT:
+        elif option.default is not None:
             value = option.default
         else:
             return
@@ -442,9 +442,7 @@ def get_option(self, option_name, scope=None, default=True):
         :return: the option value or None if not set for the given scope
         """
         option = get_option(option_name)
-
-        # Default value is `None` unless `default=True` and the `option.default` is not `NO_DEFAULT`
-        default_value = option.default if default and option.default is not NO_DEFAULT else None
+        default_value = option.default if default else None
 
         if scope is not None:
             value = self.get_profile(scope).get_option(option.name, default_value)

aiida/manage/configuration/options.py

@@ -14,8 +14,6 @@
 
 __all__ = ('get_option', 'get_option_names', 'parse_option', 'Option')
 
-NO_DEFAULT = ()
-
 
 class Option:
     """Represent a configuration option schema."""
@@ -41,7 +39,7 @@ def valid_type(self) -> Any:
 
     @property
     def default(self) -> Any:
-        return self._schema.get('default', NO_DEFAULT)
+        return self._schema.get('default', None)
 
     @property
     def description(self) -> str: