Python 根据Ist列对文件进行排序，同时修改文件中的第二列

在这个文件中，我有两列，其中Ist列描述浮点时间，第二列表示字符串

<time> <str>
8.45 A
6.12 B
10.12 C
1.45 D
2.12 E
0.45 F
....
...
..

请帮忙！我解决这个问题的方法是：我把每一列分别放在两个列表中。我用浮点数对数组进行排序，同时进行必要的调整。

我没有看到“同时修改第二列”部分。

with open('filename.txt', 'w') as fout:
     for line in sorted(open('filename.txt', 'r').readlines(), key=lambda x:float(x.split()[0])):
         fout.write(line)

with open('filename.txt', 'w') as fout:
     for line in sorted(open('filename.txt', 'r').readlines(), key=lambda x:float(x.split()[0])):
         fout.write(line)

li = ['Do you know how to read a file ?',
      'Do you know how to split a string into a list ?',
      'Do you know how to sort a list ?',
      'Do you know how to join string elements of a list ?',
      'Do you know how to write in a file ?']

def your_answer(e):
    y = 'go'
    while y not in ('yes','no'):
        y = raw_input('%s\nenter yes or no :' % e)
    return y

if any(your_answer(e)=='no' for e in li):
    print 'Please, read the documentations.'
else:
    print 'Show what you tried.'