[PYTHON] [Django] A brief summary of the log output function so that even beginners can understand it.

Introduction

Django's Logging feature can be difficult to understand at first glance. Especially the configuration file. I will summarize it easily and concisely.

Production environment assumption configuration file

It's a log setting, but if you don't set it yourself (if there is no LOGGING definition in settings.py) [Default settings included in the Django source](https://github.com/django/django/ blob / master / django / utils / log.py) will be used. However, the output log is not very easy to see, so in most cases you set it yourself. The following is a setting example assuming a production environment. Add the following contents to settings.py.

settings.py

LOGGING = {
    'version': 1,
    'disable_existing_loggers': False,
    #Log output format settings
    'formatters': {
        'production': {
            'format': '%(asctime)s [%(levelname)s] %(process)d %(thread)d '
                      '%(pathname)s:%(lineno)d %(message)s'
        },
    },
    #Handler settings
    'handlers': {
        'file': {
            'level': 'INFO',
            'class': 'logging.FileHandler',
            'filename': '/var/log/{}/app.log'.format(PROJECT_NAME),
            'formatter': 'production',
        },
    },
    #Logger settings
    'loggers': {
        #A logger that picks up logs for all applications you add
        '': {
            'handlers': ['file'],
            'level': 'INFO',
            'propagate': False,
        },
        #A logger that picks up all the logs that Django itself outputs
        'django': {
            'handlers': ['file'],
            'level': 'INFO',
            'propagate': False,
        },
    },
}

Example of use

views.py

import logging

logger = logging.getLogger(__name__)
logger.info("log info test!")
logger.error("log error test!")

The log has 5 levels according to its importance. You can output the log by letting logger execute the method with the same name as the log level as in the above usage example.

Let's look at the main setting items

disable_existing_loggers In the case of Ture, all loggers not defined in the configuration file will be disabled. At this stage, you may be wondering "What is a logger that is not defined in the configuration file?", But the meaning is that all loggers with names that are not defined in the ** logger ** item described later are disabled. Will be done. ** Basically False is OK **, but it has been verified in detail in this article, so please check if you are interested.

formatters Formatting of log output. If you set it properly here, the log output format will be aligned and the log will be easier to see, so you can think that ** setting is required **. What do% (asctime) and% (levelname) mean in Official Documents in Japanese It is described in an easy-to-understand manner.

handlers This is the setting for the log output method.

• level
  Logs below the log level set here will not be output.
• class
  This class is used for log output. In this example, the class to be output to the file is specified. If you want to rotate log files (you want the files to be separated by file size or date), it is convenient to use'logging.handlers.RotatingFileHandler' or'logging.handlers.TimedRotatingFileHandler' (the actual method is [this article]]. (It is described in detail at https://marsquai.com/745ca65e-e38b-4a8e-8d59-55421be50f7e/05f253f8-c11b-4c91-8091-989eb2600a7b/b2deae90-5afc-4bcf-9159-722dfd1aadec/). Alternatively, for example, if you specify'logging.StreamHandler', it will be output to the console.
• filename
  You can specify the path and file name of the file to be logged.
• formatter
  Specifies which of the formatters defined earlier to use.

loggers This is the logger setting that is actually used from the app. The logger defined by "''" is a description that follows the output example.

import logging
logger = logging.getLogger(__name__)

You can get it from each application. Basically, it's enough to have this and the logger settings that django itself outputs.

• handlers
  Specifies which of the handlers defined earlier to use.
• level
  Similar to the definition in handlers, logs below the log level set here will not be output.
• propagate
  This is the content of + α, so if you are not interested in it, you can think that ** basically False is OK **.

The following is an explanation of propagate. The logger can actually have a ** namespace **. As an example, define'logA'and'logA.logB' as shown below.

    'logger': {
        'logA': {
            'handlers': ['file'],
            'level': 'INFO',
            'propagate': False,
        },
        'logA.logB': {
            'handlers': ['file'],
            'level': 'INFO',
            'propagate': False,
        },

Then get the logger from the application as follows.

views.py

import logging

logger = logging.getLogger('logA.logB.logC')
logger.info("log info test!")

At this time, logC is not defined, so ** logB is acquired ** as a result. If the specified logger does not exist, it will go up to the namespace above and get the first one found.

Here, if logB and logC are also defined and the property is True, for all logA, logB, logC

logger.info("log info test!")

Is executed. In other words, if the settings of logA, logB, and logC are the same except for the name, ** logs with exactly the same contents will be output three times **. As the name of propagate, the log output instruction is also propagated to the logger in the upper hierarchy. As a usage, set logA for file output, logB for console output, get logA if you only need the file, get logB if you want to output to the console in the development environment etc. Is it an image like that (I have never used it ...).

that's all. At the end, the content was a little complicated, but unless you do something complicated, I think that the basic settings are OK with just the content described in this article. Thank you for your hard work: cat2: