一 with
python中的with的作用是自动释放对象，即使对象在使用的过程中有异常抛出。可以使用with的类型必须实现__enter__ __exit__。我的理解是=try...finally{},在finally中调用了释放函数。
[类似与CSharp中的using(){}关键字，用来自动确保调用对象的dispose()方法，即使对象有异常抛出。C#中可以使用using{}的对象必须已经实现了IDispose接口。]
def TestWith():
with open("myfile.txt") as f:
for line in f:
print (line)
f.readline() #f is already clean up here, here will meet ValueError exception
TestWith()
with open("myfile.txt") as f:
for line in f:
print (line)
f.readline() #f is already clean up here, here will meet ValueError exception
TestWith()
在with语句执行完以后，f对象马上就被释放了。所以下面在调用f.readline()会出错。
二 with + try...except
既能让对象自动释放，又包含了异常捕获的功能。
class controlled_execution(object):
def __init__(self, filename):
self.filename = filename
self.f = None
try:
f = open(self.filename, 'r')
content = f.read()
return content
except IOError as e:
print (e)
if self.f:
print ('type:%s, value:%s, traceback:%s' % (str(type), str(value), str(traceback)))
self.f.close() def TestWithAndException():
with controlled_execution("myfile.txt") as thing:
if thing:
print(thing) #TestWithAndException()
def __init__(self, filename):
self.filename = filename
self.f = None
def __enter__(self):
try:
f = open(self.filename, 'r')
content = f.read()
return content
except IOError as e:
print (e)
def __exit__(self, type, value, traceback):
if self.f:
print ('type:%s, value:%s, traceback:%s' % (str(type), str(value), str(traceback)))
self.f.close() def TestWithAndException():
with controlled_execution("myfile.txt") as thing:
if thing:
print(thing) #TestWithAndException()
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