使用with...as....语法打开一个文件

当使用open()打开一个不存在的文件时。最后通过finally关闭文件会报错

# -*- coding: utf-8 -*-

try:
    f=open("dataaaa.txt","w")
    for each_line in f:
        print (each_line)
except OSError as reason:
    print("出错了："+str(reason))
finally:
    f.close()

程序会报错：
出错啦：[Errno 2] No such file or directory: ‘dataaaa.txt’
Traceback (most recent call last):
File “E:/Python Program/test.py”, line 19, in
f.close()
NameError: name ‘f’ is not defined

解决办法为：

# -*- coding: utf-8 -*-

try:
    f=open("dataaaa.txt","w")
    for each_line in f:
        print (each_line)
except OSError as reason:
    print("出错了："+str(reason))
finally:
    if 'f' in locals():  # 如果文件对象变量存在当前局部变量符号表的话，说明打开成功
        f.close()

为了避免忘记关闭文件，可以使用with….as….

# -*- coding: utf-8 -*-

try:
    with open("dataaaa.txt","w") as f:
        for each_line in f:
            print (each_line)
except OSError as reason:
    print("出错了："+str(reason))

输出为：
出错了：not readable

感谢参考资料：
http://bbs.fishc.com/forum.php?mod=viewthread&tid=48291&extra=page%3D1%26filter%3Dtypeid%26typeid%3D398