bugfix for misuse of bytes and str & refinement

[seekamoon]

Hi,
I found a few more bugs during use. Some of them are caused by misuse of bytes and str (e.g., to check if a bytes is in a Counter whose keys are str, which will always lead to False).
There are also some trivial yet fatal problems like dict.keys()[0] or dict.items()[0], which is illegal in Python 3.x (it should be list(dict.keys())[0]).

Btw, It looks that all problems are caused by code migration from Python 2 to 3.

Have fixed them and refined the code.

[Neo23x0]

Yeah, possible. Many thanks.
Since I'm not using the Python-based generator in my daily work anymore, I didn't notice these bugs.