计算模糊dna的dna序列可能性

Question

此代码用于扫描包含不明确的dna字符串的fasta文件，并计算有多少可能的序列。由于某种原因，它似乎只在1个核苷酸后停止，但我不知道为什么

def count_ambiguous_code(file):
    'Bepaalt het aantal mogelijke sequenties afgaande uit de locatie van de ingevoerde file'
    from Bio import SeqIO
    from Bio.Alphabet.IUPAC import ambiguous_dna
    seq_records = SeqIO.parse (file, 'fasta', alphabet = ambiguous_dna)    
    for record in seq_records:   
        record.lower()    
        a, t, g, c, u, r, y, s, w, k, m, b, d, h, v, n, = [0]*16
        for nucl in record:
            if nucl == "a":
                a += 1
            elif nucl == "t":
                t += 1
            elif nucl == "g":
                g += 1
            elif nucl == "c":
                c += 1
            elif nucl == "u":
                u += 1
            elif nucl == "r":
                r += 1
            elif nucl == "y":
                y += 1
            elif nucl == "s":
                s += 1
            elif nucl == "w":
                w += 1
            elif nucl == "k":
                k += 1
            elif nucl == "m":
                m += 1
            elif nucl == "b":
                b += 1
            elif nucl == "d":
                d += 1
            elif nucl == "h":
                h += 1
            elif nucl == "v":
                v += 1
            elif nucl == "n":
                n += 1
        possibilities = ((1**(a+t+g+c+u))*(2**(r+y+s+w+k+m))*(3**(b+d+h+v))*(4**n))
    print(record.id)
    print (possibilities)