added fields.py with ItemField class and subclasses based on Django's for new items

--HG--
extra : convert_revision : svn%3Ab85faa78-f9eb-468e-a121-7cced6da292c%40865

scrapy/trunk/scrapy/contrib/item/fields.py

@@ -0,0 +1,83 @@
+import re
+
+class ItemFieldValueError(Exception):
+    pass
+
+
+class ItemField(object):
+    def __init__(self, required=False):
+        self.required = required
+
+    def to_python(self, value):
+        pass
+
+
+class BooleanItemField(ItemField):
+    def to_python(self, value):
+        if value in (True, False): return value
+        if value in ('t', 'True', '1'): return True
+        if value in ('f', 'False', '0'): return False
+        raise ItemFieldValueError("This value must be either True or False.")
+
+
+ansi_date_re = re.compile(r'^\d{4}-\d{1,2}-\d{1,2}$')
+
+
+class DateItemField(ItemField):
+    def to_python(self, value):
+        if value is None:
+            return value
+        if isinstance(value, datetime.datetime):
+            return value.date()
+        if isinstance(value, datetime.date):
+            return value
+
+        if not ansi_date_re.search(value):
+            raise ItemFieldValueError(
+                "Enter a valid date in YYYY-MM-DD format.")
+
+        year, month, day = map(int, value.split('-'))
+        try:
+            return datetime.date(year, month, day)
+        except ValueError, e:
+            raise ItemFieldValueError("Invalid date: %s" % str(e))
+
+
+class DecimalItemField(ItemField):
+    def to_python(self, value):
+        if value is None:
+            return value
+        try:
+            return decimal.Decimal(value)
+        except decimal.InvalidOperation:
+            raise ItemFieldValueError("This value must be a decimal number.")
+
+
+class FloatItemField(ItemField):
+    def to_python(self, value):
+        if value is None:
+            return value
+        try:
+            return float(value)
+        except (TypeError, ValueError):
+            raise ItemFieldValueError("This value must be a float.")
+
+
+class IntegerItemField(ItemField):
+    def to_python(self, value):
+        if value is None:
+            return value
+        try:
+            return int(value)
+        except (TypeError, ValueError):
+            raise ItemFieldValueError("This value must be an integer.")
+
+
+class StringItemField(ItemField):
+    def to_python(self, value):
+        if isinstance(value, basestring):
+            return value
+        if value is None:
+            return value
+        raise ItemFieldValueError("This field must be a string.")
+