with open("foo.txt", "r+b") as f:
    for i, line in enumerate(f):
        if i == 1:                                
            f.write(str(t))

with open('foo.txt', 'r') as f: 
    data = f.readlines()
data[1] = '%s\n' % t
with open('foo.txt', 'w') as f: 
    f.writelines(data)