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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_sequence(self,chr=None,start=None,end=None,dir=None,rng=None):
 """get a sequence :param chr: :param start: :param end: :param dir: charcter +/- :parma rng: :type chr: string :type start: int :type end: int :type dir: char :type rng: GenomicRange :return: sequence :rtype: string """

    if rng: 
      chr = rng.chr
      start = rng.start
      end = rng.end
      dir = rng.direction
    if not start: start = 1
    if not end: end = self.fai[chr]['length']
    if not dir: dir = '+'
    if dir == '-':
      return sequence.Sequence.rc(self._seqs[chr][start-1:end])
    return self._seqs[chr][start-1:end]