python – 在上下文管理器中捕獲異常__enter __()

即使在__enter __()中有異常,也可以確保__exit __()方法被呼叫？

>>> class TstContx(object):
...def __enter__(self):
...raise Exception('Oops in __enter__')
...
...def __exit__(self, e_typ, e_val, trcbak):
...print "This isn't running"
... 
>>> with TstContx():
...pass
... 
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in __enter__
Exception: Oops in __enter__
>>>

編輯

這是儘可能接近我可以得到…

class TstContx(object):
def __enter__(self):
try:
# __enter__ code
except Exception as e
self.init_exc = e

return self

def __exit__(self, e_typ, e_val, trcbak):
if all((e_typ, e_val, trcbak)):
raise e_typ, e_val, trcbak

# __exit__ code


with TstContx() as tc:
if hasattr(tc, 'init_exc'): raise tc.init_exc

# code in context

在後視中,情景管理器可能不是最好的設計決策

喜歡這個：

import sys

class Context(object):
def __enter__(self):
try:
raise Exception("Oops in __enter__")
except:
# Swallow exception if __exit__ returns a True value
if self.__exit__(*sys.exc_info()):
pass
else:
raise


def __exit__(self, e_typ, e_val, trcbak):
print "Now it's running"


with Context():
pass

要讓程式繼續執行,而不執行上下文塊,您需要檢查上下文塊中的上下文物件,並且只有在__enter__成功時才能執行重要的操作.

class Context(object):
def __init__(self):
self.enter_ok = True

def __enter__(self):
try:
raise Exception("Oops in __enter__")
except:
if self.__exit__(*sys.exc_info()):
self.enter_ok = False
else:
raise
return self

def __exit__(self, e_typ, e_val, trcbak):
print "Now this runs twice"
return True


with Context() as c:
if c.enter_ok:
print "Only runs if enter succeeded"

print "Execution continues"

據我所知,您無法完全跳過該塊.並注意到這個上下文現在吞噬了它中的所有異常.如果您希望在__enter__成功時不要吞下異常,請在__exit__中檢查self.enter_ok,如果為True,則返回False.