def __repr__(self):
        """
        Present a string representation which includes a template traceback, so
        we can tell where this error occurred in the template, as well as in
        Python.
        """
        # Avoid importing things unnecessarily until we actually need them;
        # since this is an 'error' module we should be extra paranoid about
        # that.
        from traceback import format_list
        if self._roots:
            roots = '  ' + '\n  '.join([
                    self._formatRoot(r) for r in self._roots]) + '\n'
        else:
            roots = ''
        if self._traceback:
            traceback = '\n'.join([
                    line
                    for entry in format_list(self._traceback)
                    for line in entry.splitlines()]) + '\n'
        else:
            traceback = ''
        return (
            'Exception while flattening:\n' +
            roots + traceback +
            self._exception.__class__.__name__ + ': ' +
            str(self._exception) + '\n')

        Present a string representation which includes a template traceback, so
        we can tell where this error occurred in the template, as well as in
        Python.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/error.py