diff --git a/doc/api_value.md b/doc/api_value.md
index 08ce38d..bb288ef 100644
--- a/doc/api_value.md
+++ b/doc/api_value.md
@@ -42,7 +42,6 @@ def calc_disk_usage(path, size='bytes'):
     # do not calc if path is None
     if path is None:
         return None
-
     if size == 'bytes':
         div = 1
     else:
@@ -60,12 +59,13 @@ usage = FloatOption('usage', 'Disk usage', Calculation(calc_disk_usage,
 
 Finally add those options in option description and a Config:
 
-```
+```python
 disk = OptionDescription('disk', 'Verify disk usage', [filename, usage])
 root = OptionDescription('root', 'root', [disk])
 async def main():
     config = await Config(root)
     await config.property.read_write()
+    return config
 
 config = run(main())
 ```
@@ -109,26 +109,29 @@ returns:
 
 When you enter a value it is validated:
 
->>> try:
->>>     config.option('disk.path').value.set('/unknown')
->>> except ValueError as err:
->>>     print(err)
+```python
+async def main():
+    try:
+        await config.option('disk.path').value.set('/unknown')
+    except ValueError as err:
+        print(err)
+
+run(main())
+```
+
+returns:
+
+```
 "/unknown" is an invalid file name for "Path", this directory does not exist
-
-We can also set a :doc:`calculation` as value. For example, we want to launch previous function but with in_gb to True as second argument:
-
->>> calc = Calculation(calc_disk_usage, Params((ParamOption(filename),
-...                                             ParamValue('gigabytes'))))
->>> config.option('disk.usage').value.set(calc)
->>> config.option('disk.usage').value.get()
-622.6080360412598
+```
 
 #### Is value is valid?
 
 To check is a value is valid:
 
->>> config.option('disk.path').value.valid()
-True
+```python
+await config.option('disk.path').value.valid()
+```
 
 #### Display the default value
 
diff --git a/doc/options.md b/doc/options.md
index 0644683..1598a0d 100644
--- a/doc/options.md
+++ b/doc/options.md
@@ -471,6 +471,22 @@ from tiramisu import FilenameOption
 FilenameOption('file', 'file', '/etc/tiramisu/tiramisu.conf')
 ```
 
+## Unix file permissions: PermissionsOption
+
+Valid the representing Unix permissions is an octal (base-8) notation.
+
+```python
+from tiramisu import PermissionsOption
+PermissionsOption('perms', 'perms', 755)
+PermissionsOption('perms', 'perms', 1755)
+```
+
+This option doesn't allow (or display a warning with warnings_only):
+
+- 777 (two weak value)
+- others have more right than group
+- group has more right than user
+
 # Date option
 
 ## Date option: DateOption
diff --git a/tiramisu/autolib.py b/tiramisu/autolib.py
index 1295758..f8e3ac4 100644
--- a/tiramisu/autolib.py
+++ b/tiramisu/autolib.py
@@ -90,6 +90,7 @@ class ParamDynOption(ParamOption):
                  dynoptiondescription: 'DynOptionDescription',
                  notraisepropertyerror: bool=False,
                  raisepropertyerror: bool=False,
+                 optional: bool=False,
                  todict: bool=False,
                  ) -> None:
         super().__init__(option,
@@ -99,6 +100,7 @@ class ParamDynOption(ParamOption):
                          )
         self.suffix = suffix
         self.dynoptiondescription = dynoptiondescription
+        self.optional = optional
 
 
 class ParamSelfOption(Param):
@@ -273,13 +275,19 @@ async def manager_callback(callbk: Param,
         except PropertiesOptionError as err:
             # raise PropertiesOptionError (which is catched) because must not add value None in carry_out_calculation
             if callbk.notraisepropertyerror or callbk.raisepropertyerror:
-                raise err
+                raise err from err
             raise ConfigError(_('unable to carry out a calculation for "{}"'
-                                ', {}').format(option.impl_get_display_name(), err), err)
+                                ', {}').format(option.impl_get_display_name(), err), err) from err
         except ValueError as err:
-            raise ValueError(_('the option "{0}" is used in a calculation but is invalid ({1})').format(option_bag.option.impl_get_display_name(), err))
+            raise ValueError(_('the option "{0}" is used in a calculation but is invalid ({1})').format(option_bag.option.impl_get_display_name(), err)) from err
         except AttributeError as err:
-            raise ConfigError(_(f'unable to get value for calculating "{option_bag.option.impl_get_display_name()}", {err}'))
+            if isinstance(callbk, ParamDynOption) and callbk.optional:
+                # cannot acces, simulate a propertyerror
+                raise PropertiesOptionError(option_bag,
+                                            ['configerror'],
+                                            config_bag.context.cfgimpl_get_settings(),
+                                            )
+            raise ConfigError(_(f'unable to get value for calculating "{option_bag.option.impl_get_display_name()}", {err}')) from err
         return value
 
     async def get_option_bag(config_bag,