python-logging-demo

logging is a built-in package in Python. There are only Three files in this package.

1. __init__.py
2. config.py
3. handlers.py

The logging library takes a modular approach and offers several categories of components: loggers, handlers, filters, and formatters.

• Loggers expose the interface that application code directly uses.
• Handlers send the log records (created by loggers) to the appropriate destination.
• Filters provide a finer grained facility for determining which log records to output.
• Formatters specify the layout of log records in the final output.

A good convention to use when naming loggers is to use a module-level logger, in each module which uses logging, named as follows:

logger = logging.getLogger(__name__)

上面的一句创建logger的代码，调用了 logging库中__init__.py中的getLogger()函数, 它的实现如下：

def getLogger(name=None):
    """
    Return a logger with the specified name, creating it if necessary.

    If no name is specified, return the root logger.
    """
    if name:
        return Logger.manager.getLogger(name)
    else:
        return root

通过代码可以看到，如果getLogger()有传入name字段，则查找manager是否已经有这个name的logger，若没有传入name字段，则返回root logger

root logger是什么呢？在什么时候创建的呢？

在logging库的__init__.py中，我们可以找到如下代码：

root = RootLogger(WARNING)
Logger.root = root
Logger.manager = Manager(Logger.root)

可以看到，当我们在某一个地方 import logging时，上述代码就被执行了，也就是说 root logger就已经生成了。root logger其实本质上也是 Logger类的一个实例，只是它是一个特殊的logger。

以上代码不仅仅创建了root logger，还给Logger类新增了两个类变量——root（用户新创建的root实例初始化）和manager（直接实例化一个Manager类）

Logging from multiple modules

a common practice is to create different loggers for every module.

https://docs.python.org/3/howto/logging.html