1. logging这个类有点特别，它会在初始化时生成一个全局的对象用来存储所有logger对象，而后每次import logging，可以通过getlogger来获取特定的logger对象，从而打印出相应的log。所以它类似一个树状的结构，logging.INFO()其实就是root.INFO()，是直接用根节点来打log（根节点是在import logging时就会初始化）；

  2. logging的子节点如果没有setLevel的话，默认level为0，意思是一个无效的level，会通过如下方法来搜索其父节点，直到找到有效的level位置；

    def getEffectiveLevel(self):
    """
    Get the effective level for this logger.

    Loop through this logger and its parents in the logger hierarchy,
    looking for a non-zero logging level. Return the first one found.
    """
    logger = self
    while logger:
        if logger.level:
            return logger.level
        logger = logger.parent
    return NOTSET

  3. logging的官方流程图： img

  4. logging默认的root节点的level是WARNING：

    root = RootLogger(WARNING)

  5. 执行某个logger.info()时，默认会call这个logger以及它所有父节点的handler，如果不想call其父节点的handler，则需要把logger.propagate设为False。这部分源代码如下：

    def callHandlers(self, record):
    """
    Pass a record to all relevant handlers.

    Loop through all handlers for this logger and its parents in the
    logger hierarchy. If no handler was found, output a one-off error
    message to sys.stderr. Stop searching up the hierarchy whenever a
    logger with the "propagate" attribute set to zero is found - that
    will be the last logger whose handlers are called.
    """
    c = self
    found = 0
    while c:
        for hdlr in c.handlers:
            found = found + 1
            if record.levelno >= hdlr.level:
                hdlr.handle(record)
        if not c.propagate:
            c = None    #break out
        else:
            c = c.parent
    if (found == 0) and raiseExceptions and not self.manager.emittedNoHandlerWarning:
        sys.stderr.write("No handlers could be found for logger"
                         " \"%s\"\n" % self.name)
        self.manager.emittedNoHandlerWarning = 1

  6. 使用logging打印出exception的stack trace：

    try:
    # do something here
except Exception, e:
    logging.error(e, exc_info=True)

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