详解Python logging调用Logger.info方法的处理过程

看到需要由子类去实现，以StreamHandler为例子：

def emit(self, record):
  """
  Emit a record.

  If a formatter is specified, it is used to format the record.
  The record is then written to the stream with a trailing newline. If
  exception information is present, it is formatted using
  traceback.print_exception and appended to the stream. If the stream
  has an 'encoding' attribute, it is used to determine how to do the
  output to the stream.
  """
  try:
   msg = self.format(record)
   stream = self.stream
   stream.write(msg)
   stream.write(self.terminator)
   self.flush()
  except Exception:
   self.handleError(record)

4.2 Handler.format(record):

 def format(self, record):
  """
  Format the specified record.

  If a formatter is set, use it. Otherwise, use the default formatter
  for the module.
  """
  if self.formatter:
   fmt = self.formatter
  else:
   fmt = _defaultFormatter
  return fmt.format(record)

如果handler有自定义的formatter就用自定义的，如果没有则用默认的Formatter的实例， 初始化元源码为：

 def __init__(self, fmt=None, datefmt=None, style='%'):
  """
  Initialize the formatter with specified format strings.

  Initialize the formatter either with the specified format string, or a
  default as described above. Allow for specialized date formatting with
  the optional datefmt argument (if omitted, you get the ISO8601 format).

  Use a style parameter of '%', '{' or '$' to specify that you want to
  use one of %-formatting, :meth:`str.format` (``{}``) formatting or
  :class:`string.Template` formatting in your format string.

  .. versionchanged:: 3.2
   Added the ``style`` parameter.
  """
  if style not in _STYLES:
   raise ValueError('Style must be one of: %s' % ','.join(
        _STYLES.keys()))
  self._style = _STYLES[style][0](fmt)
  self._fmt = self._style._fmt
  self.datefmt = datefmt

有三个参数：

style有三种：

_STYLES = {
 '%': (PercentStyle, BASIC_FORMAT),
 '{': (StrFormatStyle, '{levelname}:{name}:{message}'),
 '$': (StringTemplateStyle, '${levelname}:${name}:${message}'),

可以看出对应到：% 操作符的格式化， format方法的格式化以及Template的格式化。

Formatter的format方法源码为：

 def format(self, record):
  """
  Format the specified record as text.
  The record's attribute dictionary is used as the operand to a
  string formatting operation which yields the returned string.
  Before formatting the dictionary, a couple of preparatory steps
  are carried out. The message attribute of the record is computed
  using LogRecord.getMessage(). If the formatting string uses the
  time (as determined by a call to usesTime(), formatTime() is
  called to format the event time. If there is exception information,
  it is formatted using formatException() and appended to the message.
  """
  record.message = record.getMessage()
  if self.usesTime():
   record.asctime = self.formatTime(record, self.datefmt)
  s = self.formatMessage(record)
  if record.exc_info:
   # Cache the traceback text to avoid converting it multiple times
   # (it's constant anyway)
   if not record.exc_text:
    record.exc_text = self.formatException(record.exc_info)
  if record.exc_text:
   if s[-1:] != "
":
    s = s + "
"
   s = s + record.exc_text
  if record.stack_info:
   if s[-1:] != "
":
    s = s + "
"
   s = s + self.formatStack(record.stack_info)
 3/4   首页 上一页 1 2 3 4 下一页 尾页