为什么print（）和logger对象以意外的顺序流式传输到控制台输出？

Question

我尝试在使用两个logger对象之前和之后使用print函数，这两个对象将输出流式传输到控制台。 问题是我得到了一个非常混乱的输出，这是不符合预期的顺序。你知道吗

我试过把print（）改成logger_对象信息（）并且工作正常。你知道吗

包含意外输出的完整代码：

import logging

# Create logger objects and set  level
loggerA = logging.getLogger()
loggerA.setLevel(logging.DEBUG)

loggerB = logging.getLogger()
loggerB.setLevel(logging.DEBUG)

# Set log output format
log_format = logging.Formatter('%(asctime)s - %(levelname)s - %(name)s - %(message)s', datefmt='%m/%d/%Y - %I:%M:%S %p')

# Create a console stream handler
stream_handler = logging.StreamHandler()
stream_handler.setFormatter(log_format)

# Add handlers to the loggers
loggerA.addHandler(stream_handler)
loggerB.addHandler(stream_handler)


def main():
    for i in range(10):
        print('Starting loop number {}'.format(i))
        loggerA.info("loop number {}".format(i))
        loggerB.info("loop number {}".format(i))
        print('Finished loop number {}'.format(i))

if __name__ == '__main__':
    main()

添加loggerC对象以将print（）替换为日志信息（）：

...
loggerC = logging.getLogger()
loggerC.setLevel(logging.DEBUG)
...
def main():
    for i in range(10):
        loggerC('Starting loop number {}'.format(i))
        loggerA.info("loop number {}".format(i))
        loggerB.info("loop number {}".format(i))
        loggerC('Finished loop number {}'.format(i))
...

使用print（）意外输出的第一种情况是：

04/06/2019 - 10:10:34 AM - INFO - loggerA - loop number 1
Starting loop number 1
04/06/2019 - 10:10:34 AM - INFO - loggerB - loop number 1
04/06/2019 - 10:10:34 AM - INFO - loggerA - loop number 2
04/06/2019 - 10:10:34 AM - INFO - loggerB - loop number 2
Finished loop number 1
Starting loop number 2
Finished loop number 2
Starting loop number 3
Finished loop number 3
04/06/2019 - 10:10:34 AM - INFO - loggerA - loop number 3
04/06/2019 - 10:10:34 AM - INFO - loggerB - loop number 3

第二种情况是使用日志信息（）而不是预期输出的print（）：

04/06/2019 - 10:12:21 AM - INFO - loggerC - Starting loop number 1
04/06/2019 - 10:12:21 AM - INFO - loggerA - loop number 1
04/06/2019 - 10:12:21 AM - INFO - loggerB - loop number 1
04/06/2019 - 10:12:21 AM - INFO - loggerC - Finished loop number 1
04/06/2019 - 10:12:21 AM - INFO - loggerC - Starting loop number 2
04/06/2019 - 10:12:21 AM - INFO - loggerA - loop number 2
04/06/2019 - 10:12:21 AM - INFO - loggerB - loop number 2
04/06/2019 - 10:12:21 AM - INFO - loggerC - Finished loop number 2
04/06/2019 - 10:12:21 AM - INFO - loggerC - Starting loop number 3
04/06/2019 - 10:12:21 AM - INFO - loggerA - loop number 3
04/06/2019 - 10:12:21 AM - INFO - loggerB - loop number 3
04/06/2019 - 10:12:21 AM - INFO - loggerC - Finished loop number 3

Answer 1

您的问题可能是standard streams的问题。默认情况下，print使用std::out，logger.info使用std::err。两个流are bound to your terminal但是它们可能有不同的flush刷新或触发器。你知道吗

这就是为什么输出混合在一起，函数写入不同的流，它们以不同的方式刷新到终端，导致明显不一致的结果。你知道吗

将第一个代码更改为强制logger.info使用与print相同的流来解决问题：

import sys
stream_handler = logging.StreamHandler(sys.stdout)

现在，它返回正确的输出：

  Starting loop number 0
04/06/2019 - 07:41:34 AM - INFO - root - loop number 0
04/06/2019 - 07:41:34 AM - INFO - root - loop number 0
  Finished loop number 0
  Starting loop number 1
04/06/2019 - 07:41:34 AM - INFO - root - loop number 1
04/06/2019 - 07:41:34 AM - INFO - root - loop number 1
  Finished loop number 1
  Starting loop number 2
04/06/2019 - 07:41:34 AM - INFO - root - loop number 2
04/06/2019 - 07:41:34 AM - INFO - root - loop number 2
  Finished loop number 2
  Starting loop number 3
04/06/2019 - 07:41:34 AM - INFO - root - loop number 3
04/06/2019 - 07:41:34 AM - INFO - root - loop number 3
  Finished loop number 3

因为print和logger.info以正确的顺序将相同的流刷新到终端，所以结果是正确的。你知道吗

您还可以保留different streams并强制std::out流为flushed explicitly：

def main():
    for i in range(10):
        print('  Starting loop number {}'.format(i))
        # Force std::out stream (fed by print) to be flushed to the terminal
        # before logger feeds std::err and also flushes
        sys.stdout.flush() 
        loggerA.info("loop number {}".format(i))
        loggerB.info("loop number {}".format(i))
        print('  Finished loop number {}'.format(i))
        sys.stdout.flush()

第二个版本也会产生预期的结果。你知道吗

还要记住，根据定义，logger是thread-safe，但print不是。如果要使用线程创建模块，请仅使用logger跟踪执行。你知道吗