生信脚本练习（12）：计算Fasta文件中各序列长度并绘制统计图表

    import os
    import getpass
    import matplotlib.pyplot as plt
    usr = getpass.getuser()
    os.chdir('c:/Users/' + usr + '/Desktop')
    seq_len = {}
    # 把fasta文件全部读取做成字典，键是带‘>’的那一行，值是序列
    def readfasta(filename):
    fa = open(filename, 'r')
    res = {}
    ID = ''
    for line in fa:
        if line.startswith('>'):           
            ID = line.strip('\n')
            res[ID] = ''
        else:           
            res[ID] += line.strip('\n')
    return res
    seq = readfasta('Test1.fa')
    
    # 做成另外一个字典字典，键是带‘