register_line_magic

@register_line_magic
def pb(line):
    idx, title = line.split(maxsplit=1)
    if not title:
        title = None
    try:
        idx = int(idx)
    except ValueError:
        return 'First argument is not an integer'
    try:
        code = In[idx]
    except IndexError:
        return 'CellIndex does not exist.'
    return pastebin.paste_python(code, title)