title: String format a JSON string gives KeyError
body: <p>Why does this code give a <code>KeyError</code>?</p>
  
  <pre><code>output_format = """
  { 
      "File": "{filename}", 
      "Success": {success}, 
      "ErrorMessage": "{error_msg}", 
      "LogIdentifier": "{log_identifier}" 
  }
  """
  
  print output_format.format(filename='My_file_name',
                             success=True,
                             error_msg='',
                             log_identifier='123')
  </code></pre>
  
  <p>Error message:</p>
  
  <pre><code>KeyError: ' "File"'
  </code></pre>