详解Python logging调用Logger.info方法的处理过程

本次分析一下Logger.info的流程

1. Logger.info源码:

 def info(self, msg, *args, **kwargs):
  """
  Log 'msg % args' with severity 'INFO'.

  To pass exception information, use the keyword argument exc_info with
  a true value, e.g.

  logger.info("Houston, we have a %s", "interesting problem", exc_info=1)
  """
  if self.isEnabledFor(INFO):
   self._log(INFO, msg, args, **kwargs)

注释中反应了可以通过 msg和不定参数args来进行日志的格式化。
真实的调用为：_log方法：

2. Logger._log方法:

 def _log(self, level, msg, args, exc_info=None, extra=None, stack_info=False):
  """
  Low-level logging routine which creates a LogRecord and then calls
  all the handlers of this logger to handle the record.
  """
  sinfo = None
  if _srcfile:
   #IronPython doesn't track Python frames, so findCaller raises an
   #exception on some versions of IronPython. We trap it here so that
   #IronPython can use logging.
   try:
    fn, lno, func, sinfo = self.findCaller(stack_info)
   except ValueError: # pragma: no cover
    fn, lno, func = "(unknown file)", 0, "(unknown function)"
  else: # pragma: no cover
   fn, lno, func = "(unknown file)", 0, "(unknown function)"
  if exc_info:
   if isinstance(exc_info, BaseException):
    exc_info = (type(exc_info), exc_info, exc_info.__traceback__)
   elif not isinstance(exc_info, tuple):
    exc_info = sys.exc_info()
  record = self.makeRecord(self.name, level, fn, lno, msg, args,
         exc_info, func, extra, sinfo)
  self.handle(record)

最后两行：

生成日志记录：

record = self.makeRecord(self.name, level, fn, lno, msg, args, exc_info, func, extra, sinfo)

处理日志记录

self.handle(record)

2 生成日志记录:

 def makeRecord(self, name, level, fn, lno, msg, args, exc_info,
     func=None, extra=None, sinfo=None):
  """
  A factory method which can be overridden in subclasses to create
  specialized LogRecords.
  """
  rv = _logRecordFactory(name, level, fn, lno, msg, args, exc_info, func,
        sinfo)
  if extra is not None:
   for key in extra:
    if (key in ["message", "asctime"]) or (key in rv.__dict__):
     raise KeyError("Attempt to overwrite %r in LogRecord" % key)
    rv.__dict__[key] = extra[key]
  return rv

调用_logRecordFactory初始化一个日志记录实例，_logRecordFactory 其实就是LogRecord类，初始化时，可能包含logger的name, level、调用的函数、行号、日志字符串、模板参数、堆栈信息等。

再看extra信息，extra到底有何用？现在从代码中可以看到，只是更新到生成的日志记录实例的__dict__中去.猜测：肯定会在生成最终的日志字符串的时候会用到。继续往下看。

3 处理日志记录self.handle(record):

Logger继承自Filterer,

 def handle(self, record):
  """
  Call the handlers for the specified record.

  This method is used for unpickled records received from a socket, as
  well as those created locally. Logger-level filtering is applied.
  """
  if (not self.disabled) and self.filter(record):
   self.callHandlers(record)

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