python中的__enter__ __exit__

我們前面文章介紹了迭代器和可迭代對象，這次介紹python的上下文管理。在python中實現了__enter__和__exit__方法，即支持上下文管理器協議。上下文管理器就是支持上下文管理器協議的對象，它是為了with而生。當with語句在開始運行時，會在上下文管理器對象上調用 __enter__ 方法。with語句運行結束后，會在上下文管理器對象上調用 __exit__ 方法

with的語法:

with EXPR as VAR:
 BLOCK

這是上面語法的偽代碼:

mgr = (EXPR)   
exit = type(mgr).__exit__  # Not calling it yet
value = type(mgr).__enter__(mgr)    
exc = True     
try:
    try:
        VAR = value  # Only if "as VAR" is present
        BLOCK
    except:
        # The exceptional case is handled here
        exc = False
        if not exit(mgr, *sys.exc_info()):
            raise
        # The exception is swallowed if exit() returns true
finally:
    # The normal and non-local-goto cases are handled here
    if exc:
        exit(mgr, None, None, None)

1、生成上下文管理器mgr
2、如果沒有發現__exit__, __enter__兩個方法，解釋器會拋出AttributeError異常
3、調用上下文管理器的 __enter__() 方法
4、如果語法里的as VAR沒有寫，那么 偽代碼里的 VAR= 這部分也會同樣被忽略
5、如果BLOCK中的代碼正常結束，或者是通過break, continue ,return 來結束，__exit__()會使用三個None的參數來返回
6、如果執行過程中出現異常，則使用 sys.exc_info的異常信息為參數調用 __exit__(exc_type, exc_value, exc_traceback)


之前我們對文件的操作是這樣的:

try:
    f = open('filename')
except:
    print("Unexpected error:", sys.exc_info()[0])
else:
    print(f.readlines())
    f.close()

現在有了with語句可以使代碼更加簡潔，減少編碼量,下面的語句會在執行完后自動關閉文件(即使出現異常也會)。:

with open('example.info', 'r') as f:
    print(f.readlines())

一個例子:

class TmpTest:
    def __init__(self,filename):
        self.filename=filename
    def __enter__(self):
        self.f = open(self.filename, 'r')
       # return self.f
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.f.close()

test=TmpTest('file')

with test as t:
    print ('test result: {}'.format(t))

返回:

test result: None

這個例子里面__enter__沒有返回，所以with語句里的"as t"到的是None，修改一下上面的例子:

class TmpTest:
    def __init__(self,filename):
        self.filename=filename
    def __enter__(self):
        self.f = open(self.filename, 'r')
        return self.f
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.f.close()

test=TmpTest('file')

with test as t:
    print ('test result: {}'.format(t))

返回:

test result: <_io.TextIOWrapper name='file' mode='r' encoding='cp936'>

 
如果在__init__或者__enter__中拋出異常，則不會進入到__exit__中:

class TmpTest:
    def __init__(self,filename):
        self.filename=filename
        print("__init__")
        raise ImportError
    def __enter__(self):
        self.f = open(self.filename, 'r')
        print("__enter__")
        return self.f
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("__exit__")
        self.f.close()

test=TmpTest('file')
with test as t:
    print ('test result: {}'.format(t))

返回:

__init__
Traceback (most recent call last):
  File "D:/pythonScript/leetcode/leetcode.py", line 14, in <module>
    test=TmpTest('file')
  File "D:/pythonScript/leetcode/leetcode.py", line 5, in __init__
    raise ImportError
ImportError

如果在__exit__中返回True,則不會產生異常:

class TmpTest:
    def __init__(self,filename):
        self.filename=filename
        print("__init__")

    def __enter__(self):
        self.f = open(self.filename, 'r')
        print("__enter__")
        return self.f

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("__exit__ {} ".format(exc_type))
        self.f.close()
        return True

test=TmpTest('file')
with test as t:
    print ('test result: {}'.format(t))
    raise ImportError
print("no error")

返回:

__init__
__enter__
test result: <_io.TextIOWrapper name='file' mode='r' encoding='cp936'>
__exit__ <class 'ImportError'>
no error

參考: https://python3-cookbook.readthedocs.io/zh_CN/latest/c08/p03_make_objects_support_context_management_protocol.html?highlight=with
        https://docs.python.org/3/library/stdtypes.html#typecontextmanager
        https://www.python.org/dev/peps/pep-0343/