上下文管理協議

class Open:
    def __init__(self,filename):
        self.filename = filename
    def __enter__(self):
        print("__enter__")
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("__exit__")


with Open('a.txt') as f:      # with Open會觸發__enter__, __enter__返回的值會賦值給f ,而後會執行with裏面的代碼塊，最後會執行__exit__
    print(f)
    print(f.filename)
    print("============>")
    print("============>")
    print("============>")
    print("============>")

print('111111111111111111111')

返回結果：

__enter__                                              # with Open觸發__enter__的執行，把返回值給了f
<__main__.Open object at 0x0000000000B9DC18>           # 打印了f的內存地址
a.txt                                                  # 打印了f.filename
============>                                          # 打印了4個print
============>
============>
============>
__exit__                                               # with裏面執行完畢會觸發__exit__的執行
111111111111111111111                                  # 最後會執行print('111111111111111111111')

來咱們看下另一種狀況:

class Open:
    def __init__(self,filename):
        self.filename = filename
    def __enter__(self):
        print("__enter__")
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("__exit__")

print(exc_type) print(exc_val) print(exc_tb)

with Open('a.txt') as f:      
    print(f.filename)
    print(dkasfklafl)
    print("============>")
    print("============>")
print("00000000")

執行結果:

Traceback (most recent call last):
   File "E:/seafile backup/python3/python3 project/day 28/s1 上下文管理協議.py", line 43, in <module>
     print(dkasfklafl)
 NameError: name 'dkasfklafl' is not defined
__enter__                                      # with Open會觸發__enter__,把返回值賦值給f
a.txt                                          # 打印f.filename
__exit__                                       # 打印print(dkasfklafl)，由於沒有這個變量，報錯，直接觸發__exit__
<class 'NameError'>                            # 打印一個錯誤類
name 'dkasfklafl' is not defined               # 提示變量名沒有定義
<traceback object at 0x0000000000B89748>       # 追蹤信息   注意：報錯以後，下面的代碼都不會運行了

                                            # 好比：print("============>") print("============>") print("00000000")都沒有運行

會報錯，如今咱們解決剛剛的報錯

class Open:
    def __init__(self,filename):
        self.filename = filename
    def __enter__(self):
        print("__enter__")
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("__exit__")
        print(exc_type)
        print(exc_val)
        print(exc_tb)
        

return True

with Open('a.txt') as f:
    print(f)
    print(dkasfklafl)
    print("============>")
    print("============>")
print("00000000")

執行結果

__enter__                                             # with Open觸發__enter__, __enter__返回值賦值給f
<__main__.Open object at 0x000000000105DD68>          # 執行print(f)
__exit__                                              # 碰到print(dkasfklafl)異常，觸發__exit__
<class 'NameError'>                                   # print(exc_type) 打印錯誤類型                                         
name 'dkasfklafl' is not defined                      # print(exc_val) ，打印提示信息，提示變量名沒有定義
<traceback object at 0x0000000001069748>              # print(exc_tb) ，打印追蹤信息  ，以後就跳出with的代碼塊，執行下面的代碼
00000000                                              # 執行print("00000000")