运行python生成器清理代码的最佳方法 [英] best way to run python generator cleanup code

问题描述

我正在尝试编写一个生成器函数，该函数将行从数据库中取出并一次返回一行.但是，我不确定下面标记为**的清理代码是否按照我的想法执行.如果没有，将清理代码放入最后一个yield语句之后执行的生成器本身的最佳方法是什么?我看着捕获StopIteration，但这似乎是由调用者完成的，而不是在生成器中完成的.

I'm trying to write a generator function that gets rows out of a database and returns them one at a time. However, I'm not sure if the cleanup code marked ** below executes as I think it does. If it does not, what is the best way to put cleanup code inside a generator itself that executes after the last yield statement? I looked at catching StopIteration but that seems to be done from the caller, not within the generator.

def MYSQLSelectGenerator(stmt):
...
try:   
    myDB = MySQLdb.connect(host=..., port=..., user=..., passwd=..., db=...)   
    dbc=myDB.cursor()
    dbc.execute(stmt)
    d = "asdf"
    while d is not None:
        d = dbc.fetchone() #can also use fetchmany() to be more efficient
        yield d
    dbc.close() #** DOES THIS WORK AS I INTEND, MEANING AS SOON AS d = "None"
except MySQLdb.Error, msg:
    print("MYSQL ERROR!")
    print msg

推荐答案

您可以使用上下文管理器和with语句. contextlib 提供

You can use a context manager and the with statement. contextlib provides closing:

from contextlib import closing

myDB = MySQLdb.connect(host=..., port=..., user=..., passwd=..., db=...)   
with closing(myDB.cursor()) as dbc:
    dbc.execute(stmt)
    d = "asdf"
    while d is not None:
        d = dbc.fetchone() #can also use fetchmany() to be more efficient
        yield d

这将在with块的结尾处自动调用dbc上的close()，即使已引发异常.

This will automatically call close() on dbc at the end of the with block, even if an exception has been raised.

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