方法很简单，用count就可以了，

with open (r'D:\Rosalind\rosalind_dna.txt','r') as f:    for nucleotide in f.readlines():        num_A = nucleotide.count("A")        num_G = nucleotide.count("G")        num_C = nucleotide.count("C")        num_T = nucleotide.count("T")        print ("The numbers of 'A' ====> " + str(num_A))print ("The numbers of 'G' ====> " + str(num_G))print ("The numbers of 'C' ====> " + str(num_C))print ("The numbers of 'T' ====> " + str(num_T))