filename tab-completion in Cmd.cmd of Python

I'm working on making a command-line tool using Cmd.cmd of Python, and I want to add a "load" command with filename argument, which is supporting tab-completion.

Referring this and this, I mad a code like this:

import os, cmd, sys, yaml
import os.path as op
import glob as gb

def _complete_path(path):
    if op.isdir(path):
        return gb.glob(op.join(path, '*'))
    else:
        return gb.glob(path+'*')

class CmdHandler(cmd.Cmd):

    def do_load(self, filename):
        try:
            with open(filename, 'r') as f:
                self.cfg = yaml.load(f)
        except:
            print 'fail to load the file "{:}"'.format(filename)

    def complete_load(self, text, line, start_idx, end_idx):
        return _complete_path(text)

This works well for cwd, however, when I want to go into subdir, after subdir/ then the "text" of complete_load function becomes blank, so _complete_path func returns cwd again.

I don't know how to get the contents of subdir with tab-completion. Please help!