class RowError(Exception):
	""" error - a row is too long """
	def __init__(self, values):
		Exception.__init__(self, values)

class reader:
	""" reads relational data from a tsv reader """
	def __init__(self, reader):
		self._reader = reader
		self._names = reader.read()
		self._degree = len(self._names)
	def read(self):
		values = self._reader.read()
		if not values:
			return None
		if len(values) != self._degree:
			raise RowError(values)
		pairs = map(None, self._names, values)
		row = {}
		for name, value in pairs:
			row[name] = value
		return row
	def names(self):
		return self._names
	def degree(self):
		return self._degree

class writer:
	""" writes relational data to a tsv reader """
	def __init__(self, writer, names):
		self._writer = writer
		self._names = names
		self._degree = len(names)
		writer.write(names)
	def write(self, row):
		if row == None:
			self._writer.write(None)
			return
		values = []
		n = 0
		for name in self._names:
			values.append(row[name])
			n = n + 1
		if n < self._degree:
			raise RowError(row)
		self._writer.write(values)
	def names(self):
		return self._names
	def degree(self):
		return self._degree

class cut:
	""" applies a name-map to rows of a relation """
	def __init__(self, pairs):
		self._pairs = pairs
	def cut(self, row):
		out = {}
		for n1, n2 in self._pairs:
			if row.has_key(n1):
				value = row[n1]
				if out.has_key(n2) and value != out[n2]:
					return None
				out[n2] = value
		return out