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 ‘$’ to specify that you want to use one of %-formatting, :meth:str.format ({}) formatting or :class:string.Template formatting in your format string. .. versionchanged:: 3.2 Added the style parameter. """ if style not in _STYLES: raise ValueError(‘Style must be one of: %s’ % ‘,’.join( _STYLES.keys())) self._style = _STYLES[style]0 self._fmt = self._style._fmt self.datefmt = datefmt有三个参数：fmt: 格式化模板datefmt: 时间格式化参数style: 日志格式化的样式。style有三种：_STYLES = { ‘%’: (PercentStyle, BASIC_FORMAT), ‘{’: (StrFormatStyle, ‘{levelname}:{name}:{message}’), ‘$’: (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。
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