'Args:
file (str): if this file exists, will open a shell.
shell (str): one of \'ipython\', \'pdb\''

def __init__(self, file='INJECT_SHELL.tmp', shell='ipython'):

    self._file = file
    assert (shell in ['ipython', 'pdb'])
    self._shell = shell
    logger.info("Create    a    file    '{}'    to    open    {}    shell.".format(file, shell))
