code: def _get_linewise_report(self):
  		
  		d = defaultdict(list)  # line: [] of errors
  
  		for error, lines in self.errors.items():
  			for line_num in lines:
  				d[line_num].append(error)
  
  		return '\n'.join([
  			'{:>3} → {}'.format(line, error.string)
  			for line in sorted(d.keys())
  			for error in d[line]])
docs: Returns a report each line of which comprises a pair of an input line
  		and an error. Unlike in the standard report, errors will appear as many
  		times as they occur.
  
  		Helper for the get_report method.