##### Row filters

class Filter:

    '''Wraps a reader, filtering each row'''

    def __init__(self, filter_, reader):

        self.reader = reader

        self.filter = filter_

    def __iter__(self): return self

    def next(self): return self.filter(self.reader.next())

std_nulls = [r'\N']

empty_nulls = [''] + std_nulls

class NullFilter(Filter):

    '''Translates special string values to None'''

    def __init__(self, reader, nulls=std_nulls):

        map_ = dict.fromkeys(nulls, None)

        def filter_(row): return [map_.get(v, v) for v in row]

        Filter.__init__(self, filter_, reader)

class StripFilter(Filter):

    '''Strips whitespace'''

    def __init__(self, reader):

        def filter_(row): return [v.strip() for v in row]

        Filter.__init__(self, filter_, reader)