解释Python的'__enter__'和'__exit__' [英] Explaining Python's '__enter__' and '__exit__'

问题描述

我在某人的代码中看到了这一点.是什么意思?

I saw this in someone's code. What does it mean?

    def __enter__(self):
        return self

    def __exit__(self, type, value, tb):
        self.stream.close()

---

from __future__ import with_statement#for python2.5 

class a(object):
    def __enter__(self):
        print 'sss'
        return 'sss111'
    def __exit__(self ,type, value, traceback):
        print 'ok'
        return False

with a() as s:
    print s


print s

推荐答案

使用这些魔术方法(__enter__，__exit__)可以实现易于使用with语句使用的对象.

Using these magic methods (__enter__, __exit__) allows you to implement objects which can be used easily with the with statement.

这个想法是，它使得构建需要执行一些清理"代码的代码变得容易(将其视为try-finally块). 此处有更多解释.

The idea is that it makes it easy to build code which needs some 'cleandown' code executed (think of it as a try-finally block). Some more explanation here.

一个有用的例子可能是数据库连接对象(一旦对应的"with"语句超出范围，它就会自动关闭连接):

A useful example could be a database connection object (which then automagically closes the connection once the corresponding 'with'-statement goes out of scope):

class DatabaseConnection(object):

    def __enter__(self):
        # make a database connection and return it
        ...
        return self.dbconn

    def __exit__(self, exc_type, exc_val, exc_tb):
        # make sure the dbconnection gets closed
        self.dbconn.close()
        ...

如上所述，将此对象与with语句一起使用(如果您使用的是Python 2.5，则可能需要在文件顶部执行from __future__ import with_statement).

As explained above, use this object with the with statement (you may need to do from __future__ import with_statement at the top of the file if you're on Python 2.5).

with DatabaseConnection() as mydbconn:
    # do stuff

PEP343-'with'语句'的文字很好也是

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