title: Python logger throws KeyError for custom logger inputs while using logger adapter
body: <pre><code>[logger_main]
  level=INFO
  handlers=file_handler
  qualname=main
  propagate=0
  
  [handler_file_handler]
  class=FileHandler
  formatter=formatter
  class=handlers.RotatingFileHandler
  maxBytes=31457280
  level=DEBUG
  args=('../log/logfile.txt',)
  
  [formatter_formatter]
  format=%(asctime)s - %(RequestNumber)s - %(funcName)-20s() - %(levelname)-10s - %(message)s
  format=%(asctime)s - %(RequestNumber)s - %(funcName)-20s() - %(levelname)-10s - %(message)s
  </code></pre>
  
  <p>In Python I am using the logger adapter to update the request number in each log written to  file:  </p>
  
  <pre><code>log = logging.LoggerAdapter(logging.getLogger('main'), {"RequestNumber" : requestNum()});  
  </code></pre>
  
  <p>Error thrown: Below is the traceback of error -</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Python27\lib\logging\__init__.py", line 861, in emit
      msg = self.format(record)
    File "C:\Python27\lib\logging\__init__.py", line 734, in format
      return fmt.format(record)
    File "C:\Python27\lib\logging\__init__.py", line 469, in format
      s = self._fmt % record.__dict__
  KeyError: 'RequestNumber'
  </code></pre>