The complexity is 2^n. if string length is very large, this method maybe very slow

I try to convert all strings to unicode by using str.decode("utf-8") but it fails.

I was wondering how you convert katakana?

But after I comment this line,
There is no UnicodeEncodeError.

Problem happens on katakana
And I use chardet.detect to detect the code infomartion, the result is 'utf-8'
So I think decode("utf-8") is correct?

I try to transfer code to "unicode" by:
key = key.decode("utf-8")
the result is
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)

But it work well on my computer. with python 2.7.
Does anyone know why?