python中的__enter__ __exit__
我們前面文章介紹了迭代器和可迭代物件，這次介紹python的上下文管理。在python中實現了__enter__和__exit__方法，即支援上下文管理器協議。上下文管理器就是支援上下文管理器協議的物件，它是為了with而生。當with語句在開始執行時，會在上下文管理器物件上呼叫 __enter__ 方法。with語句執行結束後，會在上下文管理器物件上呼叫 __exit__ 方法
with的語法:
with EXPR as VAR: BLOCK
這是上面語法的虛擬碼:
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，直譯器會丟擲AttributeError異常
3、呼叫上下文管理器的 __enter__() 方法
4、如果語法裡的as VAR沒有寫，那麼 虛擬碼裡的 VAR= 這部分也會同樣被忽略
5、如果BLOCK中的程式碼正常結束，或者是通過break, continue ,return 來結束，__exit__()會使用三個None的引數來返回
6、如果執行過程中出現異常，則使用 sys.exc_info的異常資訊為引數呼叫 __exit__(exc_type, exc_value, exc_traceback)
之前我們對檔案的操作是這樣的:
try:
f = open('filename')
except:
print("Unexpected error:", sys.exc_info()[0])
else:
print(f.readlines())
f.close()
現在有了with語句可以使程式碼更加簡潔，減少編碼量,下面的語句會在執行完後自動關閉檔案(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
這個例子裡面__enter__沒有返回，所以with語句裡的"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__中丟擲異常，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/