calculate ItemField.default on __init__

--HG--
extra : convert_revision : svn%3Ab85faa78-f9eb-468e-a121-7cced6da292c%40880

scrapy/trunk/scrapy/contrib_exp/newitem/fields.py

@@ -14,7 +14,7 @@ class ItemFieldValueError(Exception):
 class ItemField(object):
     def __init__(self, required=False, default=None):
         self.required = required
-        self._default = default
+        self.default = default or self.to_python(None)
 
     def assign(self, value):
         if hasattr(value, '__iter__'):
@@ -33,11 +33,6 @@ class ItemField(object):
         "Converts the input iterable into a single value."
         return ' '.join(value)
 
-    @property
-    def default(self):
-        "Returns the default value for this field"
-        return self._default or self.to_python(None)
-
 
 class BooleanItemField(ItemField):
     def to_python(self, value):