python:好用的 with 文法

手動清理資源占用是個很痛苦的事情，比如剛學程式設計時候，老鳥就建議：寫完open xxx 之後一定要寫一個配對兒的 close，然後再往他倆中間寫邏輯。

python 現在有個好玩的東西，利用上下文可以自動釋放掉一個對象：

class test():
    def __init__(self,msg):
        print(msg)
    def __enter__(self):
        print('Enter Object test')
    def __exit__(self, exc_type, exc_val, exc_tb):
        print('Exit Object test')

object=test('Hello')
print('***head of code block***')
with object as t:
    print('Did something here...')
print('***end of code block***')
           

運作結果如下：

Hello
***head of code block***
Enter Object test
Did something here...
Exit Object test
***end of code block***

Process finished with exit code 0           

于是，對于系統自帶的檔案操作就有了下面這樣的用法：

content=''
with open('test.txt','a',encoding='UTF-8') as f:
    f.write('Hello World\n')

with open('test.txt','r',encoding='UTF-8') as f:
    content=f.read()

print(content)           

*注意兩個f不是同一個對象

是不是很爽？幾下就搞定了O(∩_∩)O