Python 与语句
我正在尝试使用 Python 的 with 语句，我发现在下面列出的代码中，我的 __init__ 方法被调用了两次，而我的 __exit__ 方法被调用一次。这可能意味着，如果这段代码做了任何有用的事情，就会出现资源泄漏。
class MyResource:
def __enter__(self):
print 'Entering MyResource'
return MyResource()
def __exit__(self, exc_type, exc_value, traceback):
print 'Cleaning up MyResource'
def __init__(self):
print 'Constructing MyResource'
def some_function(self):
print 'Some function'
def main():
with MyResource() as r:
r.some_function()
if __name__=='__main__':
main()
这是程序的输出：
Constructing MyResource
Entering MyResource
Constructing MyResource
Some function
Cleaning up MyResource
我猜这是因为我在 with 语句中做错了什么，有效地手动调用了构造函数。我该如何纠正这个问题？
I'm experimenting with Python's with statements, and I've found that in the following code listing my __init__ method gets called twice, while my __exit__ method gets called once. This presumably means that there will be a resource leak if this code did anything useful.
class MyResource:
def __enter__(self):
print 'Entering MyResource'
return MyResource()
def __exit__(self, exc_type, exc_value, traceback):
print 'Cleaning up MyResource'
def __init__(self):
print 'Constructing MyResource'
def some_function(self):
print 'Some function'
def main():
with MyResource() as r:
r.some_function()
if __name__=='__main__':
main()
This is the program's output:
Constructing MyResource
Entering MyResource
Constructing MyResource
Some function
Cleaning up MyResource
I'm guessing it's because I'm doing something wrong in the with statement, effectively calling the constructor manually. How do I correct this?
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评论（4）
您不应从 __enter__ 返回新实例。相反，返回
self（正在调用__enter__的实例。这就是为什么__init__()被调用两次——你调用它两次，一次在 with 语句中，一次在__enter__()中，这是正确的版本：You shouldn't return a new instance from
__enter__. Instead, returnself(the instance for which__enter__is being called. That's why__init__()is called twice -- you call it twice, once in your with statement, once in__enter__(). Here's a correct version:__init__被调用两次的原因是因为您调用了它两次：一次是在
with语句中启动MyResource对象时，另一次是在with语句调用__enter__，后者创建并返回MyResource的不同实例。您的
__enter__方法应返回 <代码>自我代替。The reason
__init__is being invoked twice is since you call it twice:Once when you initiate a
MyResourceobject in thewithstatement and once more when thewithstatement invokes__enter__which in turn creates and returns a different instance ofMyResourceYour
__enter__method should returnselfinstead.我的猜测是，您不会返回 MyResource() 而是返回 self，因为 self 是已构造类的实例。
My guess is that you do not
return MyResource()butreturn self, asselfis the instance of the already constructed class.将代码更改为：
__enter__方法用于进行初始化，而不是创建实例，它已经存在（自身）。Change your code to:
The
__enter__method is there for you to do initialization, not to create your instance, it already exists (self).