批量修改文件内容

str_file=os.getcwd()
for dirpaths, dirnames, filenames in os.walk(str_file):
for filename in filenames:
if filename.endswith('.txt'):
with open(filename,'r')as f:
line= f.readlines()
for i in range(len(line)):
if 'aaa' in line[i]:
line[i]=line[i].replace('aaa','bbb')
else:
pass
open(filename,'w').writelines(line)
else:
pass