writelines() 没换行，不知为何。
f = open('1.txt', 'w')
f.write('hello')
f.write('world\n')
f.writelines ('us')
f.writelines ('ddd')
f.write('lt')
f.close()
f = open('1.txt', 'r')
str = f.read()
print(str)
f.seek(0)
str = f.tell()
print('-------------------------------------')
i=0
for str in f.readlines():
print('line %d'%(i), str, end='')
i += 1
print('\n-------------------------------------')
f.seek(0)
i=0
while 1:
str = f.readline()
if str == '':
break
print('line %d'%(i), str, end='')
i += 1
f.close()