IndentationError: inesperado unindent ¿POR QUÉ?

<code>#!/usr/bin/python
import sys
class Seq:
    def __init__(self, id, adnseq, colen):
        self.id     = id
        self.dna    = adnseq
        self.cdnlen = colen
        self.prot   = ""
    def __str__(self):
        return ">%s\n%s\n" % (self.id, self.prot)
    def translate(self, transtable):
        self.prot = ""
        for i in range(0,len(self.dna),self.cdnlen):
            codon = self.dna[i:i+self.cdnlen]
            aa    = transtable[codon]
            self.prot += aa
    def parseCommandOptions(cmdargs):
        tfname = cmdargs[1]
        sfname = cmdargs[2]
        return (tfname, sfname)
    def readTTable(fname):
        try:
            ttable = {}
            cdnlen = -1
            tfile = open(fname, "r")
            for line in tfile:
                linearr = line.split()
                codon   = linearr[0]
                cdnlen  = len(codon)
                aa      = linearr[1]
                ttable[codon] = aa
            tfile.close()
            return (ttable, cdnlen)
    def translateSData(sfname, cdnlen, ttable):
        try: 
            sequences = []
            seqf = open(seq_fname, "r")
            line = seqf.readline()
            while line:
                if line[0] == ">":
                    id = line[1:len(line)].strip()
                    seq = ""
                    line = seqf.readline()
                    while line and line[0] != '>':
                        seq += line.strip()
                        line = seqf.readline()  
                    sequence = Seq(id, seq, cdnlen)
                    sequence.translate(ttable)
                    sequences.append(sequence)
            seqf.close()
            return sequences    
    if __name__ == "__main__":
        (trans_table_fname, seq_fname) = parseCommandOptions(sys.argv)
        (transtable, colen) = readTTable(trans_table_fname)
        seqs = translateSData(seq_fname, colen, transtable)
        for s in seqs:
            print s
</code>

Dice:

<code> def translateSeqData(sfname, cdnlen, ttable):
   ^
IndentationError: unexpected unindent
</code>

¿POR QUÉ? He comprobado miles de veces y no puedo encontrar el problema. Solo he usado pestañas y no espacios. Además, a veces pide definir la clase. ¿Eso esta bien?