def __init__(self,
fields=None,
required=('name', 'description', 'url'),
whoosh_index=None):
"""
- fields : list of fields to use, or None to calculate dynamically
- required : required data (strings)
- whoosh_index : directory to keep whoosh index in
"""
self.required = set(required)
# reserved fields
self.reserved = self.required.copy()
self.reserved.update(['modified']) # last modified, a computed value
self.search = WhooshSearch(whoosh_index=whoosh_index)
# classifier fields
self._fields = fields
self.field_set = set(fields or ())
