详解Python logging调用Logger.info方法的处理过程

3.1 if语句中有一self.filter(record)的判断，看函数名，是来筛选是否要继续处理消息的，其核心源码如下：

 def filter(self, record):
  """
  Determine if a record is loggable by consulting all the filters.

  The default is to allow the record to be logged; any filter can veto
  this and the record is then dropped. Returns a zero value if a record
  is to be dropped, else non-zero.

  .. versionchanged:: 3.2

   Allow filters to be just callables.
  """
  rv = True
  for f in self.filters:
   if hasattr(f, 'filter'):
    result = f.filter(record)
   else:
    result = f(record) # assume callable - will raise if not
   if not result:
    rv = False
    break
  return rv

可以看到， 如果在handler中的filter中如果有返回为False或空，则会屏蔽对应的record,返回True或部位空的值，则会将record放行。那么我们就可以自定义自己的filter。

3.2 让Logger中所有的handles去处理record:

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):
   if lastResort:
    if record.levelno >= lastResort.level:
     lastResort.handle(record)
   elif raiseExceptions and not self.manager.emittedNoHandlerWarning:
    sys.stderr.write("No handlers could be found for logger"
         " "%s"
" % self.name)
    self.manager.emittedNoHandlerWarning = True

代码中会去循环调用当前logger的所有handlers去处理record,for循环部分,之后，如果当前的logger的propagate的值为False或空，则不向logger的父logger传递，即向上传递。

4. Handler 中的 handler(record) 部分：

def handle(self, record):
  """
  Conditionally emit the specified logging record.

  Emission depends on filters which may have been added to the handler.
  Wrap the actual emission of the record with acquisition/release of
  the I/O thread lock. Returns whether the filter passed the record for
  emission.
  """
  rv = self.filter(record)
  if rv:
   self.acquire()
   try:
    self.emit(record)
   finally:
    self.release()
  return rv

可以看到， Handler在处理record时， 会去加锁，然后调用self.emit(record)方法去处理。

4.1 emit(record)

def emit(self, record):
  """
  Do whatever it takes to actually log the specified logging record.

  This version is intended to be implemented by subclasses and so
  raises a NotImplementedError.
  """
  raise NotImplementedError('emit must be implemented '
         'by Handler subclasses')

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