[Checkins] SVN: martian/trunk/ Add a new kind of directive: OptionalValueDirective, which takes either

[Martijn Faassen]

Log message for revision 83108:
  Add a new kind of directive: OptionalValueDirective, which takes either
  zero or one argument. If no argument is supplied, the default value can
  be determined by a rule.
  

Changed:
  U   martian/trunk/CHANGES.txt
  U   martian/trunk/src/martian/directive.py
  U   martian/trunk/src/martian/directive.txt

-=-
Modified: martian/trunk/CHANGES.txt
===================================================================
--- martian/trunk/CHANGES.txt	2008-01-23 13:39:50 UTC (rev 83107)
+++ martian/trunk/CHANGES.txt	2008-01-23 13:52:27 UTC (rev 83108)
@@ -2,12 +2,16 @@
 *******
 
 0.9.3 (unreleased)
-==================
+======-===========
 
-Bug fixes
----------
+Feature changes
+---------------
 
-* ...
+* Added an OptionalValueDirective which allows the construction of
+  directives that take either zero or one argument. If no arguments
+  are given, the ``default_value`` method on the directive is
+  called. Subclasses need to override this to return the default value
+  to use.
 
 0.9.2 (2007-11-20)
 ==================

Modified: martian/trunk/src/martian/directive.py
===================================================================
--- martian/trunk/src/martian/directive.py	2008-01-23 13:39:50 UTC (rev 83107)
+++ martian/trunk/src/martian/directive.py	2008-01-23 13:52:27 UTC (rev 83108)
@@ -144,7 +144,18 @@
     def value_factory(self, value):
         return value
 
+class OptionalValueDirective(object):
+    def check_arguments(self, value=None):
+        pass
 
+    def value_factory(self, value=None):
+        if value is None:
+            return self.default_value()
+        return value
+
+    def default_value(self):
+        raise NotImplementedError
+
 class BaseTextDirective(object):
     """
     Base directive that only accepts unicode/ASCII values.

Modified: martian/trunk/src/martian/directive.txt
===================================================================
--- martian/trunk/src/martian/directive.txt	2008-01-23 13:39:50 UTC (rev 83107)
+++ martian/trunk/src/martian/directive.txt	2008-01-23 13:52:27 UTC (rev 83108)
@@ -198,3 +198,29 @@
   Traceback (most recent call last):
     ...
   GrokImportError: You can only pass interfaces to hello2.
+
+An ``OptionalValueDirective`` accepts a single value, but the value
+can be left out. If so, the value will be calculated by a function we
+pass it::
+
+  >>> from martian.directive import OptionalValueDirective
+  >>> class MyDirective(OptionalValueDirective, OnceDirective):
+  ...    def default_value(self):
+  ...       return 4 # the default if no value is given 
+  >>> hello3 = MyDirective('hello3', ClassDirectiveContext())
+
+Let's try the directive with a single value::
+
+  >>> class Test(object):
+  ...   hello3(1)
+  >>> Test.__hello3__
+  1
+
+We can also try the directive without a value::
+
+  >>> class Test(object):
+  ...   hello3()
+  >>> Test.__hello3__
+  4
+
+