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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)
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\”\n” % 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’)
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 ‘$’ to specify that you want to
use one of %-formatting, :meth:`str.format` (“{}“) formatting or
:class:`string.Template` formatting in your format string.
.. versionchanged:: 3.2
Added the “style“ parameter.
“””
if style not in _STYLES:
raise ValueError(‘Style must be one of: %s’ % ‘,’.join(
_STYLES.keys()))
self._style = _STYLES[style][0](fmt)
self._fmt = self._style._fmt
self.datefmt = datefmt
有三个参数：
fmt: 格式化模板
datefmt: 时间格式化参数
style: 日志格式化的样式。
style 有三种：
_STYLES = {
‘%’: (PercentStyle, BASIC_FORMAT),
‘{‘: (StrFormatStyle, ‘{levelname}:{name}:{message}’),
‘$’: (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:] != “\n”:
s = s + “\n”
s = s + record.exc_text
if record.stack_info:
if s[-1:] != “\n”:
s = s + “\n”
s = s + self.formatStack(record.stack_info)
看到会调用 record.getMessage(), 这里仅仅是获取我们需要的日志信息。之后会调用 s = self.formatMessage(record):
def formatMessage(self, record):
return self._style.format(record)
其实是调用了当前 style 的 format 方法，以 % 这一类型为例 PercentStyle：
class PercentStyle(object):
default_format = ‘%(message)s’
asctime_format = ‘%(asctime)s’
asctime_search = ‘%(asctime)’
def __init__(self, fmt):
self._fmt = fmt or self.default_format
def usesTime(self):
return self._fmt.find(self.asctime_search) >= 0
def format(self, record):
return self._fmt % record.__dict__
从其中的 format 方法可以看出，是针对 record 的__dict__属性中的所有参数进行格式化，这下，就清楚了之前的 extra 参数是干嘛用的了：可以在 formatter 中加入自己自定义的一些参数，如固定的用户信息等等。
之后，将最终的 message flush 到对应的 Stream 里面去就行了，就是整个流程:
请大家多多指点。
