autotag script load error : ImportError: cannot import name defaultdict

When i use autocmd to update ctags file(http://vim.wikia.com/wiki/Autocmd_to_update_ctags_file), I found some limitation as mentioned in this tip. So I turn to use autotag( http://www.vim.org/scripts/script.php?script_id=1343 ) to auto update tags, but when vim load this script, an error occured to me like this :
>Error detected while processing /home/absa/.vim/plugin/autotag.vim:
>line  229:
>Traceback (most recent call last):
> File "<string>", line 9, in ?
>ImportError: cannot import name defaultdict
>Press ENTER or type command to continue
I don't understand python,So I don't known what can i do for debug this error.

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