为什么 'var' % {1: 'variable'} 打印 'var' 而不是像 'var' % (1,) 那样引发异常 TypeError？

我正在调试我的调试模块，我依靠 try ... catch 来检测 TypeError 并正确格式化我的消息，然后，我注意到当传递字典时，Python 不会引发传统的异常。

>>> 'var' % {1: 'variable'}
'var'
>>> 'var' % (1,)
Traceback (most recent call last):
  File "<string>", line 1, in <module>

这是日志模块的最小示例：

import logging

class SmartLogRecord(logging.LogRecord):

    def _getMessage(self, remaining_arguments):

        try:

            if self.args:
                remaining_arguments.append( self.msg % self.args )

            else:
                remaining_arguments.append( self.msg )

            return False

        except TypeError as error:
            last = self.args[-1]
            self.args = self.args[:-1]
            remaining_arguments.append( str( last ) )

            if len( self.args ):
                return True

            else:
                remaining_arguments.append( self.msg )
                return False

    def getMessage(self):
        """
        Return the message for this LogRecord.

        Return the message for this LogRecord after merging any user-supplied
        arguments with the message.
        """
        remaining_arguments = []
        self.msg = str( self.msg )

        while self._getMessage( remaining_arguments ): pass
        return " ".join( reversed( remaining_arguments ) )

logging.setLogRecordFactory(SmartLogRecord)

var = 'SmartLogRecord'
logging.warning('I am a', var)

dumb = {1: 'variable'}
logging.warning('I am a', dumb)

运行它你得到：

WARNING:root:I am a SmartLogRecord
WARNING:root:I am a

如您所见，最后dumb一条消息丢失了。