def __init__(self, file='INJECT_SHELL.tmp', shell='ipython'):
        """
        Args:
           file (str): if this file exists, will open a shell.
           shell (str): one of 'ipython', 'pdb'
        """
        self._file = file
        assert shell in ['ipython', 'pdb']
        self._shell = shell
        logger.info("Create a file '{}' to open {} shell.".format(file, shell))

        Args:
           file (str): if this file exists, will open a shell.
           shell (str): one of 'ipython', 'pdb'
        
tensorpack/callbacks/misc.py