从python的日志记录器中删除处理程序 [英] Removing handlers from python's logging loggers

问题描述

我正在使用Python的日志记录系统.在循环中从Logger对象删除处理程序时，我注意到了一个奇怪的行为.即，我的for循环将删除除一个处理程序之外的所有处理程序.附加调用.removeHandler会平滑删除最后一个处理程序.通话期间不会发出错误消息.

I am playing with Python's logging system. I have noticed a strange behavior while removing handlers from a Logger object in a loop. Namely, my for loop removes all but one handler. Additional call to .removeHandler removes the last handler smoothly. No error messages are issued during the calls.

这是测试代码:

import logging
import sys
logging.basicConfig()
dbg = logging.getLogger('dbg')
dbg.setLevel(logging.DEBUG)

testLogger = logging.getLogger('mylogger')
sh = logging.StreamHandler(sys.stdout)
fh = logging.FileHandler('mylogfile.log')
dbg.debug('before adding handlers: %d handlers'%len(testLogger.handlers))
testLogger.addHandler(fh)
testLogger.addHandler(sh)

dbg.debug('before removing. %d handlers: %s'%(len(testLogger.handlers), 
                                              str(testLogger.handlers)))
for h in testLogger.handlers:
    dbg.debug('removing handler %s'%str(h))
    testLogger.removeHandler(h)
    dbg.debug('%d more to go'%len(testLogger.handlers))

#HERE I EXPECT THAT NO HANDLER WILL REMAIN    
dbg.debug('after removing: %d handlers: %s'%(len(testLogger.handlers), 
                                              str(testLogger.handlers)))
if len(testLogger.handlers) > 0:
    #Why is this happening?
    testLogger.removeHandler(testLogger.handlers[0])
dbg.debug('after manually removing the last handler: %d handlers'%len(testLogger.handlers))    

我希望在循环结束时，没有处理程序会保留在testLogger对象中，但是 从下面的输出可以看出，对.removeHandler的最后一次调用显然失败了.尽管如此 对该函数的其他调用将按预期方式删除处理程序.这是输出:

I expect that at the end of the loop no handlers will remain in the testLogger object, however the last call to .removeHandler apparently fails, as can be seen from the output below. Nevertheless additional call to this function removes the handler as expected. Here is the output:

DEBUG:dbg:before adding handlers: 0 handlers
DEBUG:dbg:before removing. 2 handlers: [<logging.FileHandler instance at 0x021263F0>, <logging.StreamHandler instance at 0x021262B0>]
DEBUG:dbg:removing handler <logging.FileHandler instance at 0x021263F0>
DEBUG:dbg:1 more to go
DEBUG:dbg:after removing: 1 handlers: [<logging.StreamHandler instance at 0x021262B0>]
DEBUG:dbg:after manually removing the last handler: 0 handlers

更有趣的是，如果我将原始循环替换为以下循环，则该循环 可以按预期工作，并且循环结束时testLogger对象中没有任何处理程序. 这是修改后的循环:

More interestingly, if I replace the original loop with the following one, the loop works as expected and no handlers remain in the testLogger object at the end of the loop. Here is the modified loop:

while len(testLogger.handlers) > 0:
    h = testLogger.handlers[0]
    dbg.debug('removing handler %s'%str(h))
    testLogger.removeHandler(h)
    dbg.debug('%d more to go'%len(testLogger.handlers))

什么解释了这种行为?这是错误还是我错过了什么?

What explains this behaviour? Is this a bug or am I missing something?

推荐答案

这不是特定于记录器的行为. 从不更改(插入/删除元素)您当前正在迭代的列表.如果需要，请进行复印.在这种情况下，testLogger.handlers = []应该可以解决问题.

This isn't logger-specific behaviour. Never mutate (insert/remove elements) the list you're currently iterating on. If you need, make a copy. In this case testLogger.handlers = [] should do the trick.

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